Run aarch64 android tests on travis.

[pietro]

Closes #486.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling bf24cff on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[coveralls]

Coverage increased (+2.2%) to 93.617% when pulling 31860f7 on pietro:aarch64_android_travis into 050d616 on briansmith:master.

[pietro]

@briansmith I suspect this will work if you clear the build caches in travis. I'm sorry I haven't had time to look into improving the android sdk/ndk install to auto clear the cache.

[briansmith]

@pietro Thanks for your work on this.

Even after I cleared the build caches and whatnot, I couldn't get it to work.

I think we should try to get the emulator-based tests for AAarch64 Android running on top of the current (just changed) Android CI setup, which would require a very different PR. If we really have to resort to manually downloading the NDK and/or emulator to get this working then we'll do so, but I'd like that to be done only as a last resort.

A silver lining: I now finally understand more or less how the Android toolchain works and so I should be able to review future Android-targeted changes much faster.

With that in mind, I'm going to close this unless/until we can figure out a new PR that works reliably.

Thanks again for your amazing help with this and other CI issues! Really your help has been amazing.