Remove 1.3.0 mongo extension from travis build.

If someone is using 1.3.0 presumably they can update to 1.3.7 with zero issues.
Change travis build to use both 2.4 and 2.6 mongodb.
Add _id into the replace documents for concurrent tests.

On mongo 2.4 if the _id is not in the replace document an autogen one is used, making these test invalid.