Upgrade build to Gradle 1.8.
Fix exceptions during HTML generation.
Fix exception in PDF creation.
Fix PDF overflow so its readable.
Update sample BuildConfig.groovy to correct version.
Explain Spock changes for Grails 2.3.

This helps with issue GRAILS-10581. The upgrade notes now explain that Grails
comes with Spock support built in, so users should no longer use the Spock
plugin.

I have also added a summary list of the changes, making it easier to see all
of them in one go.