Added OS X specific commands to installation guide [ci skip]
Recommended using homebrew for installing MySQL and PostgreSQL

For MySql we need this for test 'DefaultsTestWithoutTransactionalFixtures#test_mysql_text_not_null_defaults_non_strict' to pass
Otherwise we will get
ActiveRecord::StatementInvalid: Mysql::Error: Field 'non_null_text' doesn't have a default value: INSERT INTO `test_mysql_text_not_null_defaults` VALUES ()