travis: Attempt to fix Android flakiness
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.

GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust