Defined stable version point of Doctrine.

As per @stof suggestion, updated 2.1 branch to define the stable version point for Doctrine libraries. @fabpot it's all yours! =)

```
| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
```