correct sh syntax

[netzfisch]

because POSIX sh doesn't understand == for string equality,

• though it is valid bash syntax
• we use = instead!

[joshuaogle]

Just noticed this PR was still up, sorry about that @netzfisch. This same change was made in https://github.com/thoughtbot/proteus-middleman/commit/62074d6ac781cc96c0dbd663ed1b1332e71a2d52 as part of our discussion over on #4, so if you pull again you should see it updated.

Thanks!