fix: Fix loading with global "define" set to null

It seems that some websites have a global "define" object set to null.
This caused our module loader to break when Shaka Player was loaded.

By checking for "define" to have a "function" type, rather than just
being defined at all, we can avoid this problem.

Closes shaka-project#2847

Change-Id: I645f61687b9ec298bcc51c8d128398cdad3dd3b9

build/wrapper.template.js

@@ -39,7 +39,7 @@
     for (var k in exportTo.shaka) {
       exports[k] = exportTo.shaka[k];
     }
-  } else if (typeof define != 'undefined' && define.amd) {
+  } else if (typeof define == 'function' && define.amd) {
     // AMD module loader.
     define(function(){
       return exportTo.shaka;