Update development dependencies. Closes github#18

Tests require a few markdown libraries to pass.

gollum.gemspec

@@ -31,9 +31,11 @@ Gem::Specification.new do |s|
   s.add_dependency('sanitize', "~> 1.1")
   s.add_dependency('nokogiri', "~> 1.4")
 
-  s.add_development_dependency('shoulda')
+  s.add_development_dependency('RedCloth')
   s.add_development_dependency('mocha')
   s.add_development_dependency('org-ruby')
+  s.add_development_dependency('rdiscount')
+  s.add_development_dependency('shoulda')
 
   # = MANIFEST =
   s.files = %w[
@@ -503,4 +505,4 @@ Gem::Specification.new do |s|
   # = MANIFEST =
 
   s.test_files = s.files.select { |path| path =~ /^test\/test_.*\.rb/ }
-end
+end