Python 2.6 compatibility and test on CI

There were a few trivial problems with Python 2.6.

• format string '{}' is not supported
• bytearray cannot be used as str

While we can drop 2.6 support now, IMO there isn't enough motivation yet.
With 2.7.9 enhancement for SSL, it makes sense to me to drop 2.7.8- once major distros have 2.7.9+. (jessie has 2.7.9 but trusty still has 2.7.6 amd CentOS 7, 2.7.5)

With the patch, it should be now fully compatible with 2.6.

I also added Travis build that runs python test with Python 2.6 on CentOS 6.
For some reason I had to adjust CMake script to successfully configure without C++ and C-GLib.