maven-enforcer: require Java 1.8.0 and above

[dhalperi]

Confirmed manually:

➜  DataflowJavaSDK git:(java8-require) ✗ JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.7.0_79.jdk/Contents/Home/ mvn clean install
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO] 
[INFO] Google Cloud Dataflow SDK for Java - Parent
[INFO] Google Cloud Dataflow SDK for Java - All
[INFO] Google Cloud Dataflow Java Examples - All
[INFO] Google Cloud Dataflow SDK for Java - Starter Archetype
[INFO] Google Cloud Dataflow SDK for Java - Examples Archetype
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building Google Cloud Dataflow SDK for Java - Parent 2.0.0-beta1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:3.0.0:clean (default-clean) @ google-cloud-dataflow-java-sdk-parent ---
[INFO] Deleting /Users/dhalperi/IdeaProjects/DataflowJavaSDK/target
[INFO] 
[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-java) @ google-cloud-dataflow-java-sdk-parent ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.RequireJavaVersion failed with message:
Detected JDK Version: 1.7.0-79 is not in the allowed range 1.8.0.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] Google Cloud Dataflow SDK for Java - Parent ........ FAILURE [  1.065 s]
[INFO] Google Cloud Dataflow SDK for Java - All ........... SKIPPED
[INFO] Google Cloud Dataflow Java Examples - All .......... SKIPPED
[INFO] Google Cloud Dataflow SDK for Java - Starter Archetype SKIPPED
[INFO] Google Cloud Dataflow SDK for Java - Examples Archetype SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.310 s
[INFO] Finished at: 2017-01-05T21:30:30-08:00
[INFO] Final Memory: 11M/310M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce (enforce-java) on project google-cloud-dataflow-java-sdk-parent: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

[davorbonaci]

LGTM.

Perhaps a comment this is for building the distribution, not the requirements on the distribution itself. It is expected for the build to fail on Java 7, but the output is still required to work on Java 7.