NPE on empty directory

Fixes NullPointerException when SD card is empty. 
File.listFiles() can return null.
http://developer.android.com/reference/java/io/File.html#listFiles%28%29
NPE on empty directory

Use listFiles variable instead of calling again File.listFiles
New Travis config file
Added missed license for Build Tools 19.1.0
Fix build tools version
Create AVD before install to it
Merge pull request #6 from arkty/master

NPE on empty dir