travis: Set variable for Darwin and check that.

There were a bunch of `$(uname -s) == 'Darwin'` calls which have now
been consolidated into one setting that's being checked everywhere.

Additionally one branch was inverted. We were using `if DARWIN; else`
everywhere except for in one place where the negated form was used. In
order to keep it consistent with the rest of the script the branches
have been swapped.
Damn it. Added a commit. I can squash if you prefer.