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.
Merge pull request #175 from tbroyer/supportedsourceversion

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