fix commonjs packaging detection

root.require is not always available (eg. in browserify and node) so the detection fails.  require should always be function in scope within a module, though.

If you look at travis, it has actually been [failing](https://travis-ci.org/jeromegn/Backbone.localStorage/builds/9163475#L458) recently.  It fails when the bundle is included, though, so it doesn't actually cause a test failure, it just makes phantomjs unable to load any tests.