Change dependency to render-core

[fix]

to org.xhtmlrenderer:core-renderer:R8pre2, to prevent the conflict error that could lead to use itext 2.0.8 and throw this issue: http://grails.1312388.n4.nabble.com/Grails-Rendering-Plugin-error-java-lang-NoSuchMethodError-com-lowagie-text-pdf-BaseFont-getCharBBox--td4525995.html