build: dont check submodules to build module

The only git submodule we need is ably-common, and this is only used as part of testing to create
test apps. We don't use it when building (despite it being a build check).

This check during build prevents other SDKs from depending on a branch of ably-js, as it doesn't have
the submodules, and can't init them.

This change therefore removes the submodules check when building the library, allowing SDKs to depend on branch
versions, if they want to.

Gruntfile.js

@@ -73,7 +73,7 @@ module.exports = function (grunt) {
     });
   });
 
-  grunt.registerTask('build', ['checkGitSubmodules', 'webpack:all', 'build:browser', 'build:node', 'build:push']);
+  grunt.registerTask('build', ['webpack:all', 'build:browser', 'build:node', 'build:push']);
 
   grunt.registerTask('all', ['build', 'requirejs']);