v0.2.2
Bump version number to v0.2.2 <!-- Reviewable:start --> [<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/string-cache/131) <!-- Reviewable:end -->
Bump version number to v0.2.2 <!-- Reviewable:start --> [<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/string-cache/131) <!-- Reviewable:end -->