Add test using bats

[mizunashi-mana]

We should add tests for maintenance.

I think Bats and Travis CI are good. (like https://github.com/php-build/php-build/tree/master/tests, https://github.com/rbenv/rbenv/tree/master/test and https://github.com/nodenv/nodenv/tree/master/test)

What about this?

[madumlao]

bats is used in the other *env scripts if I recall correctly. Bug me if I haven't put any this April!

[mizunashi-mana]

Thanks a lot!