Deprecate 'gem_devel_dependencies' in favor of 'gem_development_dependencies'

Though undocumented, we had specified gem_devel_dependencies as an allowed
parameter for @build, and it was supposed to correspond with
gem_development_dependencies in a gem spec. It was dumb to call it 'devel'
instead of 'development', which would have been a cleaner mapping. Here, we
deprecate this.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>