Report: using GroovyShell instead of running CLI directly

Taken from https://github.com/checkstyle/regression-tool/pull/48#discussion_r128231026

We are now using CLI way to invoke `diff.groovy` and it is better to do this by a Groovy invoker library, like `GroovyShell`.

An example of `GroovyShell`: https://github.com/Luolc/regression-tool/blob/dd02aa9019a7c02bafb028dcf7ec85e635256aa2/src/main/java/com/github/checkstyle/regression/report/ReportGenerator.java#L49.

More information about invoking Groovy script: http://groovy-lang.org/integrating.html

The reason we don't use `GroovyShell` now is stated at https://github.com/checkstyle/regression-tool/pull/48#issuecomment-315335160:
> I am impossible to change the working directory of the invoker. There are many relative path used in the script, and we would have problems when invoking it outside its parent folder. Even in CLI, we are not able to run it outside.

Example of relative path in `diff.groovy`: https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/diff.groovy#L183

The way to get the path of the script: https://stackoverflow.com/questions/1163093/how-do-you-get-the-path-of-the-running-script-in-groovy?answertab=votes#tab-top

We need to update `diff.groovy` if we want to use `GroovyShell` way.