Add pip2 and pip-2.7 support to pip plugin

plugins/pip/_pip

-#compdef pip pip3 pip-3.2 pip-3.3 pip-3.4
+#compdef pip pip2 pip-2.7 pip3 pip-3.2 pip-3.3 pip-3.4
 #autoload
 
 # pip zsh completion, based on homebrew completion