Remove restriction to version 6

The @SupportedSourceVersion(SourceVersion.RELEASE_6) tag was causing
warning when used in java 7 but everytyhing worked fine.

[javac] warning: Supported source version 'RELEASE_6' from annotation
processor 'org.adrianwalker.multilinestring.MultilineProcessor' less
than -source '1.7'
Merge pull request #1 from juanmendes/master

Remove restriction to version 6