Update .gitignore to skip other Gemfile.locks
Fix testing for 1.8.7

Later versions of 1.8.7 store hashes in random order so decoding to a
Hash means that the key ordering may not match and so the test fails
sometimes.

It's not really the focus of the tests to duplicate MultiJson so broken
the test apart.
Skip test for Ruby 2.0.0

Ruby 2.0.0 seems to happily handle any malformed JSON so the test is not
passing. The wrapper around MultiJson shouldn't really need to worry
about the same tests.
Also skip Ruby 1.8.7

1.8.7 is also picking up a JSON lib that is also stubborned refusing to
error when rubbish is passed in.
Merge pull request #6 from fog/skip_testing_2_0

Mitigate testing issues

Closes #3