Skip GWT tests by default.

The fact that they "hang" (i.e., take ~8 hours) confused the reporter of http://stackoverflow.com/q/43040327/28465, and I suspect he's not the first.
Plus, I mean, who wants to wait 8 hours? :)
I've updated our release docs to explicitly turn on GWT tests during the main test command.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=153193781

guava-gwt/pom.xml

@@ -17,7 +17,8 @@
     This project includes GWT-friendly sources.
   </description>
   <properties>
-    <gwt.test.include>**/GwtTestSuite.java</gwt.test.include>
+    <!-- We skip GWT tests by default. To run, pass -Dgwt.test.include=**/GwtTestSuite.java -->
+    <gwt.test.include>**/DoesNotMatchAnyTests.java</gwt.test.include>
     <gwt.version>2.8.0</gwt.version>
     <gwt.plugin.version>2.8.0</gwt.plugin.version>
     <gwt.logLevel>WARN</gwt.logLevel>