Update CONTRIBUTING.md (#4277)

The build no longer works with Maven 3.5.3.  If you try it, you end up with this:

[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (display-info) @ jenkins-parent ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireMavenVersion failed with message:
3.5.4+ required to use Incrementals.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] Jenkins main module 2.199-SNAPSHOT ................. FAILURE [ 44.098 s]