add spring to guard

Guardfile

 # A sample Guardfile
 # More info at https://github.com/guard/guard#readme
 
-guard 'rspec', :version => 2, :all_on_start => false, :all_after_pass => false do
+guard 'rspec', cmd:"spring rspec", :version => 2, :all_on_start => false, :all_after_pass => false do
   watch(%r{^spec/.+_spec\.rb$})
   watch(%r{^lib/(.+)\.rb$})     { |m| "spec/lib/#{m[1]}_spec.rb" }
   watch(%r{^lib/api/(.+)\.rb$})     { |m| "spec/requests/api/#{m[1]}_spec.rb" }