Bug fix when raven config is not defined

Hi,

I just made some minor changes:
- upgrade to Grails 2.2,
- bug fix when raven config is not defined.

When we install current plugin, it breaks our apps and plugins build because it requires a raven DSN in the config.
If you don't provide a DSN with a valid URL, it generates a malformed URL exception when the grails app is packaged.
We don't use Sentry in dev/test environment, so we don't need to specify any Sentry/Raven config.

To solve this issue, I made a small change in Configuration.groovy so that _active_ is set to false if no DSN are defined in config.

I've updated the version to 0.5.3-SNAPSHOT, you might want to remove the -SNAPSHOT part when you release the plugin.

Thanks!

Ben