add chruby-fish version to `-V` and `--version` output
make tests work locally on OS X and on Travis
add tests for chruby_auto
don't run bash chruby_auto twice
base chruby_auto action on RUBY_AUTO_VERSION value
run chruby_auto before every command

With recent changes, chruby_auto wrapper is smart and fast enough to run on
every command with minimal impact.

Only slowdown left is when actually switching rubies, which shouldn't happen
too often.