Add option to allow use of custom require.js from local file system

Change to allow grunt-template-jasmine-requirejs to support using a custom version of require.js.  The scenario is when there is a need for either a bleeding edge version of requirejs that is not in the vendor folder yet or a custom fork of requirejs.  In either of these cases, this pull request would allow users to enter a local file path to their custom require.js in the version option instead of a version number.