Require python 2.7 or refrain from using python 2.7 exclusive features

[larsbutler]

Some of our test code is using 2.7-only features (note the table of assert* statements here: http://docs.python.org/library/unittest.html#unittest.TestCase.debug).

Since of some of core dev team are still using python 2.6, we should either a) upgrade everyone to python 2.7 (and clearly document that we are no longer supporting python 2.6) or b) refrain from using python 2.7 exclusive features.

This is a copy of the following issue in the openquake repository: https://github.com/gem/openquake/issues/248