Move testing dependencies to "devDependencies"

This pull request puts testing-related Bower dependencies under `devDependencies`.

These dependencies can be omitted when installing by running `bower install --production`. A deployment script (the Heroku buildpack, for example) could use this flag to avoid downloading these dependencies when they're not necessary. `bower install` without the production flag will install both `dependencies` and `devDependencies`, so everything will continue to work the same during the initial project setup.