Update to release stdlib from MRI 2.2.0p0.
Set MRI stdlib revision to match MRI 2.2.0p0.
Update to release MRI 2.2 tests.
Revert "Update to release MRI 2.2 tests."

This reverts commit 7b90ff454cdd98ff0dd63c0ba8778cc865f73b0b.
use uri:classloader://META-INF/jruby.home for all cases with a jar-context

uri:classloader is the classloader whichi loaded jruby itself.
cleanup GEM_PATH when evaluating it

when populating the Gem::Specification.dirs from GEM_PATH and the found
specifications directories in the classloader, make sure the uri-like pathes
are all understood by jruby and do indeed exists.
[Truffle] Bignum do not have a singleton class.

* Duplicating the logic of hasNoSingleton() in SingletonClassNode is not worth it.
[Truffle] Mark those language spec failures we inherit from JRuby.
[Truffle] Work on $_
Merge branch 'master' into truffle-head