some tasks don't work without the dev dependancies

When the dependancies where installed with the option --no-dev
the clean-propel task had an exception :

exception 'InvalidArgumentException' with message 'The plugin "sfAtoumPlugin" does not exist

That's because the content of sfConfig::get('sf_environment') was null
and we test if the value is not "prod" before enabling dev plugins.

Now, we check explicitly if we are on dev or test environnment before
enabling them or if the folder sfAtoumPlugin exists and the environment
is not production (for the case when the environment is null)