Loosen gem dependencies

When I took over maintainership of metric_fu, I had to find working versions of all the gems that could work together.  (RubyParser 3 et al weren't released yet).  I found that even when I could reliably install metric_fu via bundler, I could not via RubyGems (gem install). So, that's part of why I locked down the versions.  I wanted to solve other problems.

For an example of an historical issue see https://github.com/metricfu/metric_fu/issues/45 and specifically https://github.com/metricfu/metric_fu/issues/45#issuecomment-12693295 I think it had something to do with ruby_parser 3.0.4 [Per @zenspider this should no longer be necessary](https://twitter.com/the_zenspider/status/358331088537526272)

This PR -> loosen gem dependencies and cross-fingers