.gitignore: ignore generated gems

`gem build` builds a pindah gem, which should be ignored.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
lib/pindah_cli.rb: use a newer DEFAULT_TARGET_VERSION

2.1 is a stinking old version of the Android SDK APIs.  Use the latest
(4.2) instead.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>