Remove need to put new vendor files in testem.json

Now "before-tests" Gulp task copies all necessary vendor files to a tmp
directory served by Testem.

Also:

Apparently you can't concatenante browserify bundles (why it seemed to
work before remains a mistery). So instead of concatenating unit tests
to a single file "tests.js", just browserify them and output them to
separate files in a temp directory served by Testem. It's all the same
really :)