Travis: Enable cpu-specific optimizations.

[c0gent]

I'm not sure how much if any speedup this gives but there are no drawbacks I'm aware of (unless lto=true, which we won't be using).

[vkomenda]

There was an issue related to that a couple of weeks ago: rust-lang/rust#53322.

...As Marc already pointed out before.

[c0gent]

Looks fixed already (usually nightly stuff is fixed pretty quickly).

[afck]

Looks good! The older commit has a different hash from the one in master, though, so it should probably first be rebased manually.

[c0gent]

Yeah those go away if you Rebase/Squash and merge but... done.