attempt to fix GPCACHE-52

This is an attempt to fix GPCACHE-52 so that the plugin works under Grails 2.4.2. It upgrades the plugin to 2.4.2 and makes it compatible with the change to the `GroovyPagesTemplateRenderer.findAndCacheTemplate`.

However, after my changes there are 7 functional changes failing that weren't before my changes. These failures occurred after I upgraded the plugin (and before I fixed the issue), so it seems that they're caused by some 2.3.9 -> 2.4.2 incompatibility, rather than my fix.

Hopefully you might find this PR useful, even though it's obviously incomplete.