travis: Attempt to fix Android flakiness
There's been some flaky runs on Travis where the Android emulator is having
problems staying alive... presumably? For example:
* https://travis-ci.org/rust-lang/rust/jobs/
186736745
This commit spawns the emulator in the same way as buildbot with `nohup` to hope
that it goes into the background successfully, followed by a `wait-for-device`
command. I'm not actually sure if this'll fix the problems we're seeing, but I
figure it can't hurt to test out.