delete user retweets when unfollowing

A bug fix for https://github.com/TwidereProject/Twidere-Android/issues/16
Merge remote-tracking branch 'upstream/master'
force a .travis.yml in master
try to build debug version only
check signing file

or otherwise the gradle will try to sign the app
and end up with an error saying some signing-related properties
are not found.
bring up emulator for tests (if any)
see if removing x86 img helps?
ignore tests

for now we pretend if it compiles everything will be fine
apply .travis.yml