add CI test for a big-endian target

[BurntSushi]

Given #1144, it seems prudent for ripgrep to get tested on a big-endian target.

I tried playing with this in #1152 and ran into errors. The memchr crate is an example with a big-endian target test that works: https://github.com/BurntSushi/rust-memchr/blob/974943c8bc840538e18d788b6682bbfcdccceb12/.travis.yml#L31-L35

[hlieberman-gov]

Though it's not really useful for "live" testing, we do get post-facto testing from Debian's build infra which has several big-endian targets that will at least tell us if we broke something horribly.

i.e., https://buildd.debian.org/status/package.php?p=rust-ripgrep.

[BurntSushi]

@hlieberman-gov Right, that's how #1144 was reported, AIUI. The point here is to catch problems before they get to the Debian folks. :-)

[BurntSushi]

This was done in da3431b, which adds a nightly-mips builder which should cover big endian.