[annotations] Specify javac source and target level to 1.7 in pom.xml

Summary:
Without these properties mvn-based compilation fails due to
not-supporting default 1.5 source/target levels.

Our make-based annotations build already use -source 1.7 and
-target 1.7.

Reviewed By: mityal

Differential Revision: D18037110

fbshipit-source-id: 83d1ed3a5

infer/annotations/pom.xml

@@ -8,6 +8,11 @@
   <description>Annotations for the Infer static analyzer</description>
   <url>http://fbinfer.com/</url>
 
+   <properties>
+     <maven.compiler.source>1.7</maven.compiler.source>
+     <maven.compiler.target>1.7</maven.compiler.target>
+   </properties>
+
   <licenses>
     <license>
       <name>MIT license</name>