Fix JSLint find files with relative paths

The way our Jenkins is run ends up with JSLint outputting paths with relative directory names in it. The current JSLint plugin cannot handle this properly; while it reports the violations, the file contents are not captured. This pull request adds conversion of these relative paths to absolute paths, allowing the file finder to capture the file.

Note that I'm not the only one running into this problem... a similar fork at https://github.com/bjornarg/violations-plugin/commit/dd23adc7b6fd2ee54a16c8999eab7d22ece4a4f1 seems to do the same thing, but handles some null cases differently.