[TravisCI] Disable testing Python 2.6. Unit test discovery doesn't work for that version.
[TravisCI] Disable testing Python 2.6. Unit test discovery doesn't
work for that version.

SRI-CSL/whole-program-llvm,travitch/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,travitch/whole-program-llvm,travitch/whole-program-llvm,SRI-CSL/whole-program-llvm,SRI-CSL/whole-program-llvm,travitch/whole-program-llvm