Tests: make sure RVM is available before using it
Tests: make sure RVM is available before using it

Checking the exit status of `command -v` should pretty much work
everywhere. See e.g. https://stackoverflow.com/a/677212/54401 for a
discussion.

Thanks to @davidcornu for reporting this and suggesting an alternate
fix in #58.

References #57.

chrisk/fakeweb