Ensure working java6 build

Regarding recent discussions, SoapUI is still supposed to support Java 6.

As developpers often use Java 7 to build SoapUI, some dependencies to newly Java 7 api can be added without any notice. Then, the build fails with Java 6 and a fix is lately applied (see 6831b103db78eb8bc969cc5b30a5bc9d2e460133 for instance)

This pull request adds a new check in the build (thanks to the animal-sniffer-maven-plugin) to be sure that the SoapUI code does not depends on Java 7 api.
In addition, another check is performed to be sure that dependencies does not require Java 7.

// cc @erikryverling