Update NuPIC Travis-CI config

Fixes numenta/nupic#901 as well as removes official support for 2.6 in the Travis-CI configuration.  This downloads, builds, and installs from source Python, setuptools, pip, and the NuPIC python dependencies and uses the fresh python installation as the basis for building and installing NuPIC rather than the Travis-CI system python or virtualenv.  This is due to the need to compile (and use at runtime) the libpython2.7.so shared lib that is no longer available in the build environment as of recently.