Downgrade maven-compiler-plugin to version 3.1

It fails when using annotation processor without cleaning the build
every time you run it.

    [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.2:compile     (default-compile) on project send-notification: Compilation failure: Compilation failure:
    [ERROR]  /Users/jcgay/dev/send-notification/send-notification/target/generated-sources/annotations/fr/jcgay/notification/notifier/growl/AutoValue_GrowlConfiguration.java:[7,7] duplicate class:  fr.jcgay.notification.notifier.growl.AutoValue_GrowlConfiguration

https://jira.codehaus.org/browse/MCOMPILER-235