I did not manage to get it to work by using easy_install pip or updating the pip configuration file /usr/local/bin/pip.
Instead, I removed pip and installed the distribution required by the conf file:
Uninstalling pip:
$ sudo apt purge python-pip
or $ sudo yum remove python-pip
Reinstalling required distribution of pip (change the distribution accordingly):
$ sudo easy_install pip
==9.0.3