activate enforce plugin

[jangalinski]

maven-enforcer was configured in pluginManagement but not activated. This leads to errors when using the versions plugin:

[INFO]
[ERROR] Project does not define required minimum version of Maven.
[ERROR] Update the pom.xml to contain maven-enforcer-plugin to
[ERROR] force the Maven version which is needed to build this project.
[ERROR] See https://maven.apache.org/enforcer/enforcer-rules/requireMavenVersion.html
[ERROR] Using the minimum version of Maven: 3.2.5
[INFO]

Solution: activate plugin as default.