Hard require definitions.

Instead of automatically loading the built-in definition files using the
ConstantLoader class these files are now hard required, always. This adds about
200 miliseconds to the bootup time (about 1ms per call) but it removes the need
for a lot of code complexity. Hard requires also fix errors in blowing the
stack when trying to resolve constant proxies (most notable in the Rails spec).
Re-generated all Rails definitions.

These are now based on the latest Rails 4 version.