chore(bower): move purecss to devDependencies

Now that pure-grids is being bundled with encore-ui.css, purecss should no longer be listed as a dependency

NOTE: This has the potential for being a breaking change if an app has mishandled its dependencies
and is relying on encore-ui to install purecss.


FYI, validated by checking that the grids demo page stayed the same after `rm -rf demo/bower_components` then `bower install`