Can't update core with composer: "Your requirements could not be resolved to an installable set of packages" I am not able to perform a security upgrade of Drupal core with composer.  I get "requirements could not be resolved."  Here is my composer.json:
https://pastebin.com/KxEQdPkt

Here is the response:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for symfony/psr-http-message-bridge v0.2 -&gt; satisfiable by symfony/psr-http-message-bridge[v0.2].
    - drupal/core 8.3.2 requires symfony/psr-http-message-bridge ^1.0 -&gt; satisfiable by symfony/psr-http-message-bridge[1.0.x-dev].
    - drupal/core 8.3.3 requires symfony/psr-http-message-bridge ^1.0 -&gt; satisfiable by symfony/psr-http-message-bridge[1.0.x-dev].
    - drupal/core 8.3.x-dev requires symfony/psr-http-message-bridge ^1.0 -&gt; satisfiable by symfony/psr-http-message-bridge[1.0.x-dev].
    - drupal/core 8.4.x-dev requires symfony/psr-http-message-bridge ^1.0 -&gt; satisfiable by symfony/psr-http-message-bridge[1.0.x-dev].
    - Conclusion: don't install symfony/psr-http-message-bridge 1.0.x-dev
    - Installation request for drupal/core ^8.3.2 -&gt; satisfiable by drupal/core[8.3.2, 8.3.3, 8.3.x-dev, 8.4.x-dev].
The solution in my case was to delete the vendor directory and delete composer.lock.

I am not totally sure I needed to delete the vendor directory, because I tried that first without deleting composer.lock, and it didn't work.  Deleting composer.lock was necessary.

I also changed the minimum-stability to "stable" before deleting vendor and composer.lock, but I doubt that was necessary.  In any case, I now have...


  "minimum-stability": "stable",