do not define rspec task if rspec is unavailable

lib/tasks/tests.rake

@@ -11,9 +11,11 @@ namespace :ci do
   end
 end
 
-namespace :tests do
-  desc "Run all specs that generate fixtures for rspec or jasmine"
-  RSpec::Core::RakeTask.new(:generate_fixtures) do |t|
-    t.rspec_opts = ['--tag fixture']
+if defined?(RSpec)
+  namespace :tests do
+    desc "Run all specs that generate fixtures for rspec or jasmine"
+    RSpec::Core::RakeTask.new(:generate_fixtures) do |t|
+      t.rspec_opts = ['--tag fixture']
+    end
   end
 end