Consolidate javac version handling in pom

MOE_MIGRATED_REVID=144328366

Author: cushon

pom.xml

@@ -109,7 +109,7 @@
       <dependency>
         <groupId>com.google.errorprone</groupId>
         <artifactId>javac</artifactId>
-        <version>9-dev-r3297-1</version>
+        <version>${javac.version}</version>
       </dependency>
 
       <!-- Compile-time dependencies -->