add tests for rehash, whence, which
undo `assert_output_lines` in tests

It was a dumb idea and it wasn't even implemented perfectly.
add tests for version commands
add tests for `help` and `rbenv --version`
Merge branch 'test-coverage' for MAXIMUM COVERAGE
improve `versions` tests by cd'ing into a clean slate
restore original IFS for hooks
test that IFS in hooks is correct

closes #379