Update font-kit to v0.14 and freetype-sys to v0.20

Closes issue #93 (closed). Marking as a draft until the image transition (#91 (closed)) is completed, so that plotters-bitmap and plotters can be touched again.

What needs to be done:

1. unmark plotters-bitmap's image feature tests as FLAKY
2. bump plotters' font-kit dependency to v0.14

I've already tested 1. and 2. above on top of the image transition.

Ready to merge.

/cc @werdahias