Fix code for python < 2.7.8.
Fix code for python < 2.7.8.

https://github.com/tobgu/pyrsistent/issues/27

lukemarsden/flocker,1d4Nf6/flocker,achanda/flocker,jml/flocker,adamtheturtle/flocker,Azulinho/flocker,w4ngyi/flocker,wallnerryan/flocker-profiles,moypray/flocker,AndyHuu/flocker,runcom/flocker,hackday-profilers/flocker,hackday-profilers/flocker,wallnerryan/flocker-profiles,adamtheturtle/flocker,LaynePeng/flocker,moypray/flocker,adamtheturtle/flocker,agonzalezro/flocker,lukemarsden/flocker,mbrukman/flocker,agonzalezro/flocker,AndyHuu/flocker,mbrukman/flocker,wallnerryan/flocker-profiles,hackday-profilers/flocker,runcom/flocker,jml/flocker,lukemarsden/flocker,w4ngyi/flocker,LaynePeng/flocker,Azulinho/flocker,1d4Nf6/flocker,jml/flocker,moypray/flocker,w4ngyi/flocker,achanda/flocker,1d4Nf6/flocker,AndyHuu/flocker,LaynePeng/flocker,achanda/flocker,runcom/flocker,agonzalezro/flocker,mbrukman/flocker,Azulinho/flocker