Fix postgresql builds

`pg` 1.0 is not compatible with current Rails version

Author: tvdeyen

Gemfile

@@ -14,7 +14,7 @@ end
 group :test do
   gem 'sqlite3' if ENV['DB'].nil? || ENV['DB'] == 'sqlite'
   gem 'mysql2'  if ENV['DB'] == 'mysql'
-  gem 'pg'      if ENV['DB'] == 'postgresql'
+  gem 'pg', '~> 0.21' if ENV['DB'] == 'postgresql'
   gem 'simplecov', require: false
   if ENV['TRAVIS']
     gem "codeclimate-test-reporter", '~> 1.0', require: false