Allow a bit of variance due to fp precision issues.

Update test harness to allow for a bit of variance in the
results due to floating point precision issues. The test for
'52w' is off by one second when using -ffast-math or SSE.

This is no big thing; allow it.
Re-enable clang.

Now that we're not depending on -fmath=387, go ahead and
enable clang in travis-ci again.