Fix callback calling when using from a parent directory

Authors might want to use a shared reveal.js installation for all their
presentations such as:

	$ ls -1 -F
	20120105-how-to-use-git.html
	20121101-wikimedia-scaling.html
	reveal.js/
	$

In this case, the plugin callbacks will not be called at all.

When using head.js, the callback is marked as depending upon the loading of a
Javscript filename. The regex used to find out the filename is applied to the
full path which in the above case would be something like:

	reveal.js/plugin/highlight/highlight.js

The regex will thus give out 'reveal.js' as a file depency instead of the
expected 'highlight.js'

The fix is quiet easy: simply make sure that we are looking for a file that
actually ends with '.js' instead of simply containing '.js' by adding a $.