Update devDependencies to latest

Bumps our `devDependencies` to their latest versions. Notes of interest:

* load-grunt-tasks. The `multimatch` dep includes a breaking [change](https://github.com/sindresorhus/multimatch/releases/tag/v0.3.0), which doesn't appear to apply to us.
* grunt-concurrent: Our version (0.5) is from March. The current stable is 1.0
* grunt-contrib-compass: primarily dependency bumps