Fix travis.yml to install unittest2 in Python 2.6
Fix travis.yml to install unittest2 in Python 2.6

PyTables/PyTables,avalentino/PyTables,jennolsen84/PyTables,FrancescAlted/PyTables,jack-pappas/PyTables,andreabedini/PyTables,mohamed-ali/PyTables,gdementen/PyTables,joonro/PyTables,joonro/PyTables,rabernat/PyTables,tp199911/PyTables,andreabedini/PyTables,jennolsen84/PyTables,joonro/PyTables,tp199911/PyTables,joonro/PyTables,mohamed-ali/PyTables,mohamed-ali/PyTables,dotsdl/PyTables,rdhyee/PyTables,jack-pappas/PyTables,gdementen/PyTables,dotsdl/PyTables,rdhyee/PyTables,avalentino/PyTables,FrancescAlted/PyTables,FrancescAlted/PyTables,jack-pappas/PyTables,rdhyee/PyTables,tp199911/PyTables,PyTables/PyTables,tp199911/PyTables,jack-pappas/PyTables,gdementen/PyTables,dotsdl/PyTables,jennolsen84/PyTables,jack-pappas/PyTables,rabernat/PyTables,dotsdl/PyTables,rabernat/PyTables,mohamed-ali/PyTables,avalentino/PyTables,andreabedini/PyTables,andreabedini/PyTables,gdementen/PyTables,PyTables/PyTables,rabernat/PyTables,jennolsen84/PyTables,rdhyee/PyTables