Don't install executables via Rubygems
Don't install executables via Rubygems

Rubygems should be used to install dependencies, not to distribute
command-line tools (other than perhaps those highly-coupled to a
specific Ruby version). Our packaging with Traveling Ruby allows us to
distribute Parity via popular package managers or for users to compile
and install locally.

This change removes the gemspec statement that installs the
`development`, `production`, and `staging` executables through Rubygems.
Those files remain available in packaging, and will be installed by apt
and Homebrew.

Close #77.

thoughtbot/parity,gabebw/parity,thoughtbot/parity,gabebw/parity