Fixes Underscore 1.5.0 _.bindAll(this) in Testem

- the 1.5.0 release of underscore removed the ability to use _.bindAll
without enumerating any method names; this commit is the intended way
to get the same behaviour as documented in https://github.com/documentcloud/underscore/commit/bf657be243a075b5e72acc8a83e6f12a564d8f55