I faced the same problem with the same error message but on Ubuntu 16.04 LTS (Xenial Xerus) instead:
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-w71uo1rg/poster/
I tested all the solutions provided above and none of them worked for me. I read the full TraceBack and found out I had to create the virtual environment with Python version 2.7 instead (the default one uses Python 3.5 instead):
virtualenv --python=/usr/bin/python2.7 my_venv
Once I activated it, I run pip install unirest
successfully.