Use Object.const_get when defined rather than checking RUBY_VERSION

```safe_constantize``` eventually ends up poking around in the autoloading code, which means stat'ing files and various things, which is very slow.  JRuby 1.7 has Object.const_get and RUBY_VERSION == '1.9.3', so we should probably check whether it exists and use it if so, rather than testing for RUBY_VERSION.