prevent osx_defaults booleans from applying every time

`/usr/bin/defaults read` returns 0 / 1 for booleans, but
`/usr/bin/defaults write` only accepts true/false or yes/no as input, so
we need to map between input and the value we check cause our unless
condition to work properly.

closes #17
Merge pull request #19 from phinze/fix-osx-defaults-booleans

prevent osx_defaults booleans from applying every time