Disable doclint when building under JDK8.

Because it complains about everything and fails the build.

Also fix something that my IDE was marking as an error: the <overWrite> element from the configuration for the maven-dependency-plugin isn't valid for the unpack-dependencies goal (there's <overWriteIfNewer>, but it's true by default), so I removed it.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82468188
Removed test on url(). It's partially testing ClassLoader, and it sometimes fails.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82525887
Specify source level 1.6 for GWT.

This fixes failures in guava-gwt when building/testing under JDK8.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82526985