Bump Pillow requirement to >= 2.3.0, < 3.0.0

Avoiding the [FreeType >= 2.5.1 incompatibility][1] and giving Pillow
the benefit of the doubt for [semantic versioning][2].  Details in the
commit messages.

This PR is based on #369, so you probably want to merge/reject that
first.

[1]: https://github.com/thumbor/thumbor/issues/248
[2]: http://semver.org/