Upgrade to Maven Enforcer plugin 1.1

Release Notes - Maven 2.x Enforcer Plugin - Version 1.1

** Bug
   * [MENFORCER-98] - requirePluginVersions rule is not compatible with maven 3.0-beta-1
   * [MENFORCER-114] - Typo in Require Plugin Versions Documentation
   * [MENFORCER-125] - requireProperty documentation's example for property.version uses wrong regex

** Improvement
   * [MENFORCER-118] - DependencyConvergence gets better if it doesn't fail on snapshots of same baseVersion

** New Feature
   * [MENFORCER-128] - Fail the build if a dependency is overwriten with an incompatible lower version (patch)