Update home of DoctrineExtensions.

As you can read in the 2014-03-20 DoctrineExtensions update: https://github.com/Atlantic18/DoctrineExtensions/blob/master/README.md#latest-updates

>
> DoctrineExtensions has new home on github under an unbrella of ORM designer organization.
>
minor #4186 Update home of DoctrineExtensions. (rubenrua)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4186).

Discussion
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Update home of DoctrineExtensions.

As you can read in the 2014-03-20 DoctrineExtensions update: https://github.com/Atlantic18/DoctrineExtensions/blob/master/README.md#latest-updates

>
> DoctrineExtensions has new home on github under an unbrella of ORM designer organization.
>

Commits
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75c88a3 Update home of DoctrineExtensions.