7953: Fix Docker image to build CentOS 6.6 packages.

7953: Fix Docker image to build CentOS 6.6 packages.

* Follow the Software Collections documentation at
  <https://wiki.centos.org/AdditionalResources/Repositories/SCL>.
  This downgrades us to Python 3.3, but that shouldn't be a
  problem, since we test and build packages for Debian 7 which uses
  Python 3.2.  On the plus side, it removes an external dependency
  on softwarecollections.org.

* The python-pip package disappeared.  I don't know where it went.
  Maybe there were changes to the underlying centos:6 Docker image?
  Install it manually.