Upgrade JRuby version and fix windows pathing issues

This pull request fixes a number of known issues when using this plugin in a windows environment.

Jenkins issue [12094](https://issues.jenkins-ci.org/browse/JENKINS-12094) where the plugin fails to initialize with the error "haml gem cannot be found" was due to a bug in Jruby 1.7.5.  Upgrading to JRuby 1.7.16 also resolves [23071](https://issues.jenkins-ci.org/browse/JENKINS-23071).

Jenkins issue [25782](https://issues.jenkins-ci.org/browse/JENKINS-25782) is due to the usage of the URL datatype when attempting to resolve jar paths.  The use of this datatype causes any spaces in windows paths to be escaped by default and replaced by ASCII hexidecimal spaces => %20.  classesJar.exists() will return false for these escaped paths and therefore the classes.jar will never be expanded as expected.  These ASCII spaces are manually being replaced for the moment but could be upgraded in the future to use the URLDecoder.decode(classesJar.getAbsolutePath(), "ASCII") statement.
This fix also addresses issue [18528](https://issues.jenkins-ci.org/browse/JENKINS-18528).