Move dev requirements to gemspec, not Gemfile

This is more consistent with the style of developing webfontloader as a
packaged gem that's included by other projects. This also updates all of the
versions of development gems to recent versions.

Gemfile

@@ -1,10 +1,3 @@
-source :gemcutter
+source :rubygems
 
-gem "rake"
-gem "jasmine"
-
-group :demos do
-  gem "rack"
-  gem "sinatra"
-  gem "vegas"
-end
+gemspec

webfontloader.gemspec

@@ -56,9 +56,11 @@ DESC
 
   ## List your development dependencies here. Development dependencies are
   ## those that are only needed during development
-  s.add_development_dependency('rack', ["~>1.2.1"])
-  s.add_development_dependency('sinatra', ["~>1.0"])
-  s.add_development_dependency('vegas', ["~>0.1.6"])
+  s.add_development_dependency('rake')
+  s.add_development_dependency('rack', ["~>1.5.1"])
+  s.add_development_dependency('sinatra', ["~>1.3.4"])
+  s.add_development_dependency('vegas', ["~>0.1.11"])
+  s.add_development_dependency('jasmine', ["~>1.3.1"])
 
   ## Leave this section as-is. It will be automatically generated from the
   ## contents of your Git repository via the gemspec task. DO NOT REMOVE