Fixed issue with rendering colors on Lollipop

For some reason, a BitmapShader with a backing Bitmap of size 1x1 renders
incorrectly on Lollipop. The easy solution is to make 2x2 the minimum size
when converting a Drawable to a Bitmap.
Merge pull request #62 from dlew/fix-color-drawables

Fixed issue with converting ColorDrawables on Lollipop