Auto merge of #131 - KiChjang:v0.2.2, r=jdm
Auto merge of #131 - KiChjang:v0.2.2, r=jdm

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 -->

servo/string-cache