title: Can't install pg gem on Rails
body: <p>I am trying to bundle install, but, for some strange reason, the pg gem is returning the following error on install:</p>
  
  <pre><code>$ gem install pg -v '0.18.4'
  Building native extensions.  This could take a while...
  ERROR:  Error installing pg:
      ERROR: Failed to build gem native extension.
  
      /Users/username/.rvm/rubies/ruby-2.2.1/bin/ruby -r ./siteconf20151221-23315-1tkv3fd.rb extconf.rb
  checking for pg_config... no
  No pg_config... trying anyway. If building fails, please try again with
   --with-pg-config=/path/to/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
      --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=/Users/username/.rvm/rubies/ruby-2.2.1/bin/$(RUBY_BASE_NAME)
      --with-pg
      --without-pg
      --enable-windows-cross
      --disable-windows-cross
      --with-pg-config
      --without-pg-config
      --with-pg_config
      --without-pg_config
      --with-pg-dir
      --without-pg-dir
      --with-pg-include
      --without-pg-include=${pg-dir}/include
      --with-pg-lib
      --without-pg-lib=${pg-dir}/lib
  
  extconf failed, exit code 1
  
  Gem files will remain installed in /Users/username/.rvm/gems/ruby-2.2.1/gems/pg-0.18.4 for inspection.
  Results logged to /Users/username/.rvm/gems/ruby-2.2.1/extensions/x86_64-darwin-15/2.2.0/pg-0.18.4/gem_make.out
  </code></pre>
  
  <p>Can anyone please help me install this gem and make my bundle finish successfully?</p>