Update Install instructions for 2.6 users

[hskarlupka]

Setuptools stopped supporting python 2.6 in v37. Update install docs with a CentOS 6 section to tell people to:
pip install setuptools==36.8.0
before installing pyglidein.

[hugovk]

Do you still need to support Python 2.6? It's EOL, hasn't received security updates in 4 years and generally has low use.

Here's monthly pip installs from PyPI (via pypinfo --percent --pip --markdown "" pyversion)

python_version	percent	download_count
2.7	77.7%	435,636,261
3.6	8.8%	49,199,570
3.5	8.4%	46,927,461
3.4	3.9%	21,843,444
2.6	1.1%	6,194,026
3.3	0.1%	554,661
3.7	0.1%	428,159
3.2	0.0%	38,570
None	0.0%	21,921
2.8	0.0%	35
3.1	0.0%	26

[dsschult]

Yes, because RHEL6 runs python 2.6. Unfortunately, it still is the most popular OS on clusters.

[hskarlupka]

Updated Install Readme.