Travis: Django 1.7rc2; explicit build matrix
Travis: Django 1.7rc2; explicit build matrix

Switch to explicitly declaring build matrix, with the versions
of Python supported by each version of Django we handle.
(Exclusion list was becoming too complicated.)

brack3t/Djrill,idlweb/Djrill,janusnic/Djrill,barseghyanartur/Djrill,janusnic/Djrill,barseghyanartur/Djrill,idlweb/Djrill