Add armv7-linux-androideabi target to CI

[grovesNL]

We should update our Travis configuration to run CI on the armv7-linux-androideabi target (see #28)

[Lokathor]

We also need an entry for armv7-unknown-linux-gnueabihf, the raspberry pi family. We also might as well add other targets with a stdlib.

There's no actual blocker for this change but it's a non trivial set of file changes to the CI that should all go into their own PR for clarity. Instead of adding one line to the script, we'd re-arrange it all so that the build matrix got a lot bigger and we'd run a lot more builds in semi-parallel.

@MaulingMonkey set this all up in the travis for lokacore, and it should be fairly easy to copy that over here (along with all its support script files).

[grovesNL]

The Android targets are in CI now.