title: rails 4.2.0: can't install pg gem on ubuntu 14.04
body: <p>I have recently tried to upgrade my rail 3.2* application to rails 4.2.*. But, I was stopped while installing the <code>'pg'</code> gem. When I googled it, the solutions which are mostly related to OSX only. But I'm using ubuntu 14.04. Need any suggestion for the below issue.</p>
  
  <p>I have installed <code>postgresql 9.3.5</code> version on my machine. </p>
  
  <pre><code>Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
  
      /usr/local/rvm/rubies/ruby-2.1.2/bin/ruby extconf.rb 
  checking for pg_config... yes
  Using config values from /usr/bin/pg_config
  checking for libpq-fe.h... no
  Can't find the 'libpq-fe.h header
  *** extconf.rb failed ***
  Could not create Makefile due to some reason, probably lack of necessary
  libraries and/or headers.  Check the mkmf.log file for more details.  You may
  need configuration options.
  
  Provided configuration options:
      --with-opt-dir
      --without-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/usr/local/rvm/rubies/ruby-2.1.2/bin/ruby
      --with-pg
      --without-pg
      --enable-windows-cross
      --disable-windows-cross
      --with-pg-config
      --without-pg-config
      --with-pg_config
      --without-pg_config
      --with-pg-dir
      --without-pg-dir
      --with-pg-include
      --without-pg-include=${pg-dir}/include
      --with-pg-lib
      --without-pg-lib=${pg-dir}/lib
  
  extconf failed, exit code 1
  
  Gem files will remain installed in /usr/local/rvm/gems/ruby-2.1.2@r4/gems/pg-0.18.1 for inspection.
  Results logged to /usr/local/rvm/gems/ruby-2.1.2@r4/extensions/x86_64-linux/2.1.0/pg-0.18.1/gem_make.out
  An error occurred while installing pg (0.18.1), and Bundler cannot continue.
  Make sure that `gem install pg -v '0.18.1'` succeeds before bundling.
  </code></pre>