Don't require() a file that was path.join()ed.

This commit fixes an issue where the SDK would not correctly
load with `require("aws-sdk")` on a Windows machine running Node.js
0.11.x. This error is caused by the following two behavioral changes
in Node.js:

* joyent/node#6829
* joyent/node#7031

In essence, Node.js `require()` calls return different objects for
differently cased filenames, even on case-insensitive file systems.
This, coupled with the fact that `path.join()` lowercases the drive
letter in the path, causes the SDK to try to load core.js two
separate times, creating two objects that do not have the
correct properties attached.

See #303 for more information.