Try that one:
ripple.xml
<?xml version="1.0" encoding="utf-8"?>
<ripple android:color="@color/COLOR_WHILE_PRESSING" xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/background"></item>
</ripple>
background.xml
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
<solid android:color="@color/BACKGROUND_COLOR" />
<corners android:radius="6dp" />
</shape>
Or maybe this post will help you: Android L FAB Button shadow
I explained there, how to implement the new FAB button, I also used the outline for that.