README: gem install methodray fails

Thank you for developing such an interesting project.

One thing I noticed: the command gem install methodray shown in README.md fails.

Steps to reproduce

$ gem install methodray
ERROR:  Could not find a valid gem 'methodray' (>= 0) in any repository
$ gem install method-ray   # this one succeeds

It seems better to either update the README to gem install method-ray, or rename the
gem to spec.name = 'methodray' and re-publish it.

update the README to gem install method-ray

I addressed it below PRs. Thanks for your feedback.