Did you declare that you want to use java 1.6 in your project pom.xml?:
<build>
<pluginManagement>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.6</source>
<target>1.6</target>
<compilerArgument></compilerArgument>
</configuration>
</plugin>
</plugins>
</pluginManagement>
</build>