Fixed issues with app.rb require

When trying to launch the app if the `LOAD_PATH` was not properly set
the unicorn server was unable to load the `app.rb` source
file. Updated to use the `expand_path` method from the File class.

config/unicorn.rb

 require 'tmpdir'
 
 # Load app.rb to get all dependencies.
-require 'app.rb'
+require File.expand_path('../../app.rb', __FILE__)
 
 # Make sure elasticsearch is configured correctly
 UnicornHelpers.exit_on_invalid_index

config/unicorn_tcp.rb

 require 'tmpdir'
 
 # Load app.rb to get all dependencies.
-require 'app.rb'
+require File.expand_path('../../app.rb', __FILE__)
 
 # Make sure elasticsearch is configured correctly
 UnicornHelpers.exit_on_invalid_index