Add failing test to illustrate GIF error

See https://github.com/matthewwithanm/django-imagekit/issues/270
Fix "bad transparency mask" error

Addresses matthewwithanm/django-imagekit#270
Merge branch 'bad-transparency-mask'

* bad-transparency-mask:
  Fix "bad transparency mask" error
  Add failing test to illustrate GIF error
Bump version to 1.1.10