Fixed SensioDistributionBundle requirements

The 3.0.12 version introduces the `removeSymfonyStandardFiles` method
in `Sensio\Bundle\DistributionBundle\Composer\ScriptHandler` which
is required for the `post-install` and `post-update` scripts.

When the cURL PHP extension is not installed, Composer will install
the 3.0.11 version of SensioDistributionBundle which don't depends on cURL,
but the scripts requiring the `removeSymfonyStandardFiles` method will
fail.

By requiring at least the 3.0.12 version, we ensure Composer will
abort with an helpful error, saying that `ext-curl` needs to be
installed.

Before:

> Method Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::removeSymfonyStandardFiles is not callable, can not call post-install-cmd script

After:

> Your requirements could not be resolved to an installable set of packages.
>
> Problem 1
>     - sensiolabs/security-checker v2.0.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
>     - sensio/distribution-bundle v3.0.12 requires sensiolabs/security-checker ~2.0 -> satisfiable by sensiolabs/security-checker[v2.0.0].
>     - Installation request for sensio/distribution-bundle ~3.0.12 -> satisfiable by sensio/distribution-bundle[v3.0.12].
minor #749 Fixed SensioDistributionBundle requirements (valeriangalliat)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #749).

Discussion
----------

Fixed SensioDistributionBundle requirements

The 3.0.12 version introduces the `removeSymfonyStandardFiles` method in `Sensio\Bundle\DistributionBundle\Composer\ScriptHandler` which is required for the `post-install` and `post-update` scripts.

When the cURL PHP extension is not installed, Composer will install the 3.0.11 version of SensioDistributionBundle which don't depends on cURL, but the scripts requiring the `removeSymfonyStandardFiles` method will fail.

By requiring at least the 3.0.12 version, we ensure Composer will abort with an helpful error, saying that `ext-curl` needs to be installed.

Before:

    Method Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::removeSymfonyStandardFiles is not callable, can not call post-install-cmd script

After:

    Your requirements could not be resolved to an installable set of packages.

    Problem 1
        - sensiolabs/security-checker v2.0.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
        - sensio/distribution-bundle v3.0.12 requires sensiolabs/security-checker ~2.0 -> satisfiable by sensiolabs/security-checker[v2.0.0].
        - Installation request for sensio/distribution-bundle ~3.0.12 -> satisfiable by sensio/distribution-bundle[v3.0.12].

Commits
-------

90c196e Fixed SensioDistributionBundle requirements