get_qiita_version should store and retrieve information from redis

[antgonza]

Currently, every time a page gets displayed we check the current git version of the qiita code via "get_qiita_version", which uses GitPython. The issue is that it actually uses Popen to do this operation which means that it issues a new command every time, which is less than ideal.

[antgonza]

While looking on how to fix this, I found this comment: https://github.com/gitpython-developers/GitPython#leakage-of-system-resources