This PR add support for Rails 4.1

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Bump Rails version to 4.1
Removes mocha and uses Minitest for stubing
Uses latest version of postgres_ext gem from branch rails4.1
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Some changes seems to be incompatible with Rails 4.1 probably there is a work around to have support for both versions.

This branch depends on branch rails4.1. from postgres_ext all tests pass also I have a project where I'm using both gems from Github and everything works just fine.