Fixes problem with current_activity on KitKat

I'm getting weird characters at the end of the string on my HTC One with KitKat when calling ``current_activity``.
Here is a quick and dirty patch.

```ruby
Felipes-MacBook-Pro:whi-android(feature/heart_multiple_collections) $ calabash-android console weheartit/build/apk/weheartit-debug-unaligned.apk
irb(main):001:0> start_test_server_in_background
nil
irb(main):002:0> current_activity
"SidebarContentActivity t144"
irb(main):003:0> current_activity.split.first
"SidebarContentActivity"
```