title: yii2 composer update error: Invalid version string "2.*"
body: <p>I'm use composer update in a <code>Yii2</code> project, but it is exiting with an exception.
  I have updated composer to the lastest version, <code>c6cc6dd6070871f4b198ed39f76dd8047c116b02</code>, but it still doesn't work:</p>
  
  <pre><code>Importing tag v2.0.2 (2.0.2.0)
  Reading bower.json of bower-asset/yii2-pjax (v2.0.1)
  Reading /home/xiaoai/.composer/cache/repo/github.com/yiisoft/jquery-pjax/bower-f07ce95f6098c0bd5421789a20789f39a19be73b from cache
  Importing tag v2.0.1 (2.0.1.0)
  Reading /home/xiaoai/.composer/cache/repo/https---packagist.org/provider-yiisoft$yii2-bootstrap.json from cache
  Reading /home/xiaoai/.composer/cache/repo/https---packagist.org/provider-bower-asset$bootstrap.json from cache
  Downloading https://bower.herokuapp.com/packages/bootstrap
  Writing /home/xiaoai/.composer/cache/repo/https---bower.herokuapp.com-packages/bootstrap-3c71cc99d2fc1c12a3d3e1b27e448ca612a89a1d-package.json into cache
  Adding VCS repository bower-asset/bootstrap
  Downloading https://api.github.com/repos/twbs/bootstrap
  Downloading https://api.github.com/repos/twbs/bootstrap/contents/bower.json?ref=master
  Downloading https://api.github.com/repos/twbs/bootstrap/commits/master
  Downloading https://api.github.com/repos/twbs/bootstrap/tags?per_page=100
  Downloading https://api.github.com/repos/twbs/bootstrap/git/refs/heads?per_page=100
  
  
  
    [UnexpectedValueException]                                              
    Could not parse version constraint &lt;=2.*: Invalid version string "2.*"  
  
  Exception trace:
   () at phar:///usr/bin/composer.phar/src/Composer/Package/Version/VersionParser.php:461
   Composer\Package\Version\VersionParser-&gt;parseConstraint() at phar:///usr/bin/composer.phar/src/Composer/Package/Version/VersionParser.php:257
   Composer\Package\Version\VersionParser-&gt;parseConstraints() at phar:///usr/bin/composer.phar/src/Composer/Package/Loader/ArrayLoader.php:234
   Composer\Package\Loader\ArrayLoader-&gt;parseLinks() at phar:///usr/bin/composer.phar/src/Composer/Package/Loader/ArrayLoader.php:123
   Composer\Package\Loader\ArrayLoader-&gt;load() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/AssetVcsRepository.php:174
   Fxp\Composer\AssetPlugin\Repository\AssetVcsRepository-&gt;preInitBranchPackage() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/AssetVcsRepository.php:138
   Fxp\Composer\AssetPlugin\Repository\AssetVcsRepository-&gt;initBranches() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/AssetVcsRepository.php:45
   Fxp\Composer\AssetPlugin\Repository\AssetVcsRepository-&gt;initialize() at phar:///usr/bin/composer.phar/src/Composer/Repository/ArrayRepository.php:178
   Composer\Repository\ArrayRepository-&gt;getPackages() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/Pool.php:104
   Composer\DependencyResolver\Pool-&gt;addRepository() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/Util.php:60
   Fxp\Composer\AssetPlugin\Repository\Util::addRepositoryInstance() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/Util.php:39
   Fxp\Composer\AssetPlugin\Repository\Util::addRepository() at /home/xiaoai/.composer/vendor/fxp/composer-asset-plugin/Repository/AbstractAssetsRepository.php:139
   Fxp\Composer\AssetPlugin\Repository\AbstractAssetsRepository-&gt;whatProvides() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/Pool.php:199
   Composer\DependencyResolver\Pool-&gt;computeWhatProvides() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/Pool.php:188
   Composer\DependencyResolver\Pool-&gt;whatProvides() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/RuleSetGenerator.php:161
   Composer\DependencyResolver\RuleSetGenerator-&gt;whitelistFromPackage() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/RuleSetGenerator.php:273
   Composer\DependencyResolver\RuleSetGenerator-&gt;whitelistFromJobs() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/RuleSetGenerator.php:324
   Composer\DependencyResolver\RuleSetGenerator-&gt;getRulesFor() at phar:///usr/bin/composer.phar/src/Composer/DependencyResolver/Solver.php:177
   Composer\DependencyResolver\Solver-&gt;solve() at phar:///usr/bin/composer.phar/src/Composer/Installer.php:505
   Composer\Installer-&gt;doInstall() at phar:///usr/bin/composer.phar/src/Composer/Installer.php:230
   Composer\Installer-&gt;run() at phar:///usr/bin/composer.phar/src/Composer/Command/UpdateCommand.php:140
  </code></pre>
  
  <p>I don't think this is my mistake, but how do I fix the exception?</p>