Add missing test files in the gemspec.

Once sinatra-contrib was packed as a gem, some rspec tests failed
because of missing files in the gemspecs.

sinatra-contrib.gemspec

@@ -66,6 +66,10 @@ Gem::Specification.new do |s|
     "spec/content_for/different_key.erubis",
     "spec/content_for/different_key.haml",
     "spec/content_for/different_key.slim",
+    "spec/content_for/footer.erb",
+    "spec/content_for/footer.erubis",
+    "spec/content_for/footer.haml",
+    "spec/content_for/footer.slim",
     "spec/content_for/layout.erb",
     "spec/content_for/layout.erubis",
     "spec/content_for/layout.haml",
@@ -105,6 +109,7 @@ Gem::Specification.new do |s|
     "spec/reloader_spec.rb",
     "spec/respond_with/bar.erb",
     "spec/respond_with/bar.json.erb",
+    "spec/respond_with/baz.yajl",
     "spec/respond_with/foo.html.erb",
     "spec/respond_with/not_html.sass",
     "spec/respond_with_spec.rb",