fix(TreeRunner): Prevents returning an empty callback for AMD-ish modules

By default, the behavior of fetch rules was to return cb('') when
the module exists. For AMD-like modules, this is undersirable since AMD
modules can be dynamic by nature. This fix protects the if checks by
ensuring we are looking at an AMD-like module before running these tests.
Merge pull request #299 from emersonyu/master

fix(TreeRunner): Prevents returning an empty callback for AMD-ish modules