Pom includes a maven-plugin as a dependency

The sonar maven plugin is defined as a project dependency once again.

First reported at https://github.com/redouane59/twittered/issues/79
Fixed with https://github.com/redouane59/twittered/commit/1f035962957e8cc7df5c43a410eab24c75dad24d

Reintroduced by https://github.com/redouane59/twittered/commit/5b7e495268647a7a924b5c418ab03a6b882035f5