Update plugin API version to SNAPSHOT-2.6

Also add maven-compiler-plugin to prevent the error:

   annotations are not supported in -source 1.3

Change-Id: Ic927780f0247cfbfabe3b32bdd95f2100cfb28dd

pom.xml

diff --git a/pom.xml b/pom.xml
index edde056..e0a1b6d 100644
--- a/pom.xml
+++ b/pom.xml
@@ -29,12 +29,20 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <Gerrit-ApiType>plugin</Gerrit-ApiType>
-    <Gerrit-ApiVersion>2.5-SNAPSHOT</Gerrit-ApiVersion>
+    <Gerrit-ApiVersion>2.6-SNAPSHOT</Gerrit-ApiVersion>
   </properties>
 
   <build>
     <plugins>
       <plugin>
+        <artifactId>maven-compiler-plugin</artifactId>
+          <version>2.3.2</version>
+          <configuration>
+            <source>1.6</source>
+            <target>1.6</target>
+          </configuration>
+      </plugin>
+      <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>
         <version>2.4</version>