switch to numerizer gem

[tardate]

Hi Aaron, I made this mod based on your fork in order to suppress const warnings when chronic is used in conjunction with chronic_duration (or other gems that use variants of chronic), by relying on the numerizer gem rather than including the numerizer source directly. Sending this pull request in case its something you'd want to include in your fork too.

There's one new test failure a a result of this change: it doesn't handle "tenth", which I still need to investigate, but appears to be due to some changes in the numerizer gem when comapared to the numerizer class from the original chronic implementation (failure at test/test_parsing.rb:651 : assert_equal parse_now("may 10th"), parse_now("may tenth")