Fix test suite when no system Ruby exists

Some tests assumed that the `ruby` executable will be found in system PATH.

Fixes #512, closes #514
Force TAP output from Bats on CI

In Travis CI environment, Bats thinks it's outputting to an interactive
terminal, so it switches to "pretty" format and ANSI escape codes which
don't look well in the final output.
Merge branch 'master' of github.com:ArtemTitoulenko/rbenv