Updating hashCode implementations.

Per discussion on the last commit's pull request [1], updating the
implementations of hashCode for RubyNil and RubyBoolean. Now the hashCode
behavior for nil and booleans will only change when consistent hashing is
enabled. Adds a hashCode instance variable to RubyBoolean and RubyNil that is
set in the constructor to the Object#hashCode value (using
System.identityHashCode) or a static value.

[1]: https://github.com/jruby/jruby/pull/590