Commit 2989ad86 exposed the `units_per_em` function from the stb_truetype
crate, but did not update the Cargo.toml to reflect that it was added
in the 0.2.2 patch of stb_truetype.

Projects already using a "0.2" semver in their Cargo.toml would not
bother to update from 0.2.0 or 0.2.1, and results in a compile error.