Fix UMD build

#### 1/3 PRs resolving https://github.com/marionettejs/backbone.marionette/issues/1264

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**Changes:**
- AMD: Return value instead of passing in exports object
- Common.js: Use `module.exports` instead of passing in exports object
- Globals: Don't pass in empty object
- Switch from concatenated build to import bare builds of wreqr and babysitter

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[See built lib files here](https://gist.github.com/thejameskyle/ecde48a3cce63c2e1aa4)