Bump Canvas version

Canvas version 1.1.1 adds support for Giflib 5 ([relevant commit](https://github.com/LearnBoost/node-canvas/commit/a0632275c53d0ed4811a96fb7bf12ebd5668f289)) which is the version available on some platforms, such as Arch Linux for instance.
Bumping the version of Canvas will make it possible to use assetgraph-sprite on those platforms without going through the trouble of installing an old version of the library.