fixed opacity

I found this bug when fixing https://github.com/flavienlaurent/tickplusdrawable/pull/2.
It seems like the older Androids render a Drawable differently:

![screenshot2](https://cloud.githubusercontent.com/assets/616399/3928987/8f9142fa-2415-11e4-8995-8db267e47a67.png)
![screenshot1](https://cloud.githubusercontent.com/assets/616399/3928986/8f8f4612-2415-11e4-98d5-43f9261d068a.png)

I am no expert on this but the [API doc](http://developer.android.com/reference/android/graphics/drawable/Drawable.html#getOpacity()) suggests a Drawable to be as conservative as possible so I changed the opacity to TRANSLUCENT and that fixed the issue.