Remove duplicate gem

Author: vasilakisfil

rubycas-server-core.gemspec

@@ -20,7 +20,6 @@ Gem::Specification.new do |gem|
   gem.add_dependency "r18n-core"
   gem.add_dependency "activesupport", ">= 3.0"
 
-  gem.add_development_dependency "rake"
   gem.add_development_dependency "rspec"
   gem.add_development_dependency "bundler"
 end