Build auto-value-annotations for Java 7.

Fixes #655

Perhaps it would make sense to split AutoFactory and AutoService into separate annotations and processor artifacts, as well, and then to compile their annotations artifacts for Java 7. But the AutoValue change solves the immediate problem.

RELNOTES: Returned to building `auto-value-annotations` for Java 7 instead of Java 8.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205730548

value/annotations/pom.xml

@@ -32,6 +32,10 @@
     Immutable value-type code generation for Java 1.6+.
   </description>
 
+  <properties>
+    <java.version>1.7</java.version>
+  </properties>
+
   <scm>
     <url>http://github.com/google/auto</url>
     <connection>scm:git:git://github.com/google/auto.git</connection>