Expand Travis PHP versions

We should run tests against other versions of PHP than just 5.4, so we
can detect issues before trying to upgrade to one of them.  I don't want
to support earlier versions, though, because then we'll lose the short
array syntax. :)
Merge expand-travis-versions (#4) into master

Expand Travis PHP versions

We should run tests against other versions of PHP than just 5.4, so we
can detect issues before trying to upgrade to one of them.  I don't want
to support earlier versions, though, because then we'll lose the short
array syntax. :)