Revert "set travis CI dist to bionic"
Revert "set travis CI dist to bionic"

This reverts commit d4b7cf71ff74691a1c8a9641ffa016743c753d7e.

whoozle/android-file-transfer-linux,whoozle/android-file-transfer-linux,whoozle/android-file-transfer-linux,whoozle/android-file-transfer-linux