Use Ruby 2.1.5

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/
Use bin/setup from Travis

In order to test the executable documentation provided by bin/setup, we
should use `bin/setup` from CI.

I transitioned checks for gem presence to use `gem list --installed` and
to omit the `--no-document` flag. This flag should be decided by the
user's `.gemrc`.

Stop redirecting STDERR.

Expand grep flags and add --fixed-strings

The expanded flags tell me what this is doing. `--fixed-strings` gives a
tiny speed boost because we're not using regex here.
v.1.20.0