Support running RLTK on Ruby 1.8.7.

I'm using RLTK for a project I'm working on which hopes to maintain
compatibility with Ruby 1.8, which necessitated making RLTK work on Ruby
1.8.

Changes:
  - Change instances of Ruby 1.9 hash syntax to hashrockets.
  - Change instances of Ruby 1.9 stabby to proc keyword.
  - Change double splat method arguments to concat + splat.
  - Explicitly rescue LoadError when loading the cg test suite.

Caveats:
  - I had to disable ParserTester#test_use on 1.8 because the
    inherited hook is called *after* class definition when using anonymous
    classes on 1.8. This is fixed on >= 1.9.
    (See: http://bugs.ruby-lang.org/issues/4273)