[MRG+1] Dropping python 2.6 support (#7890)

* Remove Python 2.6 support

Some details about some slightly orthogonal changes:
* Note about cheking safely for nan is likely not valid any more (commit
  introducing it is c80ca91b)
* scipy.linalg.qr econ parameter removed since scipy 0.9 in favour of
  mode='economic'
* Remove unnecessary libgfortran in conda create command

* Putative fix by setting the random seed

* Revert unintended change

* Reinstate previous logic for checking for NaNs

* Reinstate change in error message

Error messages from Python 2.7 assertRegexp does not contain the
function name, in contrast with Python 3 assertRegex