Adds RbConfig class to remove deprecation warnings

[kurko]

Since Ruby 1.9.1, the Config class should be renamed to RbConfig. This
commit adds that but also leaves the old Config for olders Ruby
versions.

[kurko]

I couldn't make rake work. Tried the described below with no success.

→ ruby -rubygems -Ilib -Itest -I./ test/test_watchr.rb 
Run options: --seed 19276

# Running tests:

/Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/gems/ruby-fsevent-0.2.1/lib/fsevent/signal_ext.rb:46:in `trap': wrong number of arguments (2 for 1) (ArgumentError)
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/gems/1.9.1/gems/mocha-0.11.4/lib/mocha/integration/mini_test/version_230_to_262.rb:46:in `run'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:787:in `block in _run_suite'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:780:in `map'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:780:in `_run_suite'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:770:in `block in _run_suites'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:770:in `map'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:770:in `_run_suites'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:746:in `_run_anything'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:909:in `run_tests'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:896:in `block in _run'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:895:in `each'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:895:in `_run'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:884:in `run'
        from /Users/kurko/.rbenv/versions/1.9.3-p0/lib/ruby/1.9.1/minitest/unit.rb:664:in `block in autorun'

[kevinburke]

This has been fixed here:

http://github.com/kevinburke/observr