Use SourceVersion.latestSupported() to avoid warning with JDK 7.

When using dagger-compiler in a Java 7 project, the compiler used to issue
a "Supported source version 'RELEASE_6' from annotation processor
'dagger.internal.codegen.InjectProcessor' less than -source '1.7'" warning.

See also http://stackoverflow.com/a/8188860/116472 and hibernate/hibernate-metamodelgen@d8ce150760f579a9c0499376cc8471c80d75ddda, among others (I trust JBoss more than others about using `latestSupported()` rather than `latest()`, maybe I'm wrong).