Make GroovyWebConfiguration conditional on spring-webmvc's availability

GroovyWebConfiguration creates a GroovyTemplateViewResolver which is
a UrlBasedViewResolver sub-class. UrlBasedViewResolver is provided
by spring-webmvc. Previously, if a user configured a web application
but did not have spring-webmvc on the classpath, the application
would fail to start with a NoClassDefFoundError for
UrlBasedViewResolver.

This commit makes GroovyWebConfiguration conditional on
UrlBasedViewResolver being on the classpath so that it backs of in
the absence of spring-webmvc.

Fixes gh-1793