Actually skip goals when configured to do so

[lipusal]

Git hook installation and code validation are not being skipped when configured to do so as per the README. Example reproduction:

1. Create new empty Maven project
2. Add v2.4 of the plugin as specified in the README
3. Run mvn test -Dgcf.skip=true (or otherwise configure such parameter from the POM)
4. Output will contain:

   [INFO] --- git-code-format-maven-plugin:2.4:install-hooks (install-formatter-hook) @ test --- [INFO] skipped [INFO] Installing git hooks [INFO] Installed git hooks 

   and ls .git/hooks will show the hook is indeed installed.

This PR simply adds a return statement when skip is enabled to actually skip.

[reda-alaoui]

Thank you !