README: mention pip version is outdated.
README: mention pip version is outdated.

nxnfufunezn/legit,hickford/legit,nxnfufunezn/legit,ericmjl/legit,jetgeng/legit,ericmjl/legit,hickford/legit,kennethreitz/legit,jetgeng/legit,kennethreitz/legit,wkentaro/legit,deshion/legit,wkentaro/legit,deshion/legit