Run travis with as many extensions as possible

[AdamNiederer]

No description provided.

[alexcrichton]

Thanks for the PR! I've opened up #46 which I think is the strategy we'll want to take for tests here. I think the test failures are also "legitimate" in the sense that if you've got avx2 enabled some of the sse2 operations are compiled a little more optimized than they usually are. I believe the strategy in #46 handles this and should enable vanilla cargo test to work most of the time.