Previously the build was unable to build against JDK 7, test against
JDK 7, and test against JDK 6.
This commit allows setting of a gradle property  named jdk6Home.
Upon setting the jdk6Home property tests will be ran against JDK 6 as
well. One option for setting the property is to include jdk6Home
property in ~/.gradle/gradle.properties. For example the content of
the file might look like:
An alternative is configure jdk6Home from the commandline as shown below:
gradle build -Pjdk6Home=/usr/lib/jvm/jdk1.6.0_32