POM configurado para compilar na versão 1.7.

pom.xml

<project ...>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.5.1</version>
        <configuration>
          <source>1.7</source>
          <target>1.7</target>
        </configuration>  
      </plugin>  
    </plugins>  
  </build>  
  ...
</project>