Upgrade to maven-compiler-plugin 3.3

Release notes:

https://mail-archives.apache.org/mod_mbox/maven-announce/201410.mbox/%3C543C42F9.9010709%40apache.org%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201503.mbox/%3C20150327182100.6FA0C17BE1@minotaur.apache.org%3E

pom.xml

@@ -94,7 +94,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.1</version>
+        <version>3.3</version>
         <configuration>
           <source>1.7</source>
           <target>1.7</target>