Upgrade to RSpec 3.1 and Rubocop 027.1

* implicitly require spec_helper
* randomize specs
* display verbose spec output when running just one spec

close #57 (issue)

close #63 (PR)
Add .ruby-version and .ruby-gemset to .gitignore

Folks who use those files to manage their gemsets can do so without git
offering to stage those files. h/t @vraravam