fixed 'check' task config so it can be run inside a subproject folder

otherwise it would fail due to relative file paths like this:

$ cd subprojects/groovy-test
$ gradle check
> Unable to create a Checker: cannot initialize module RegexpHeader - unable to load header file config/checkstyle/codeHeader.txt
removed unused checkstyle config.xml file

- it was replaced by checkstyle.xml
Merge pull request #257 from darxriggs/checkstyle-subprojects-fix

Checkstyle subprojects fix