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'

There are other possible solutions, see http://stackoverflow.com/questions/8185331/forward-compatible-java-6-annotation-processor-and-supportedsourceversion but this seemed to be the simplest