Rename `share/chruby/dev.sh` to `share/chruby/testing.sh` (or other)

[postmodern]

@k0kubun @eregon do you have any objections to renaming dev.sh to testing.sh. I feel like dev.sh is a little too vague of a name. Other ideas include minimal.sh or no_gems.sh. I can rename it now before releasing 0.4.0, or hold off on renaming until 1.0.0.