Updated README.md - Added RAILS_ENV

Added RAILS_ENV to gitlab:test command, so the dev database wont be wiped.

fixes https://github.com/gitlabhq/gitlabhq/issues/5756#issuecomment-29876630

README.md

@@ -102,7 +102,7 @@ or start each component separately
 
 * Run all tests
 
-        bundle exec rake gitlab:test
+        bundle exec rake gitlab:test RAILS_ENV=test
 
 * [RSpec](http://rspec.info/) unit and functional tests