more aggressive caching for travis / venv and set python version
more aggressive caching for travis / venv and set python version

jfterpstra/bluebottle,jfterpstra/bluebottle,onepercentclub/bluebottle,onepercentclub/bluebottle,jfterpstra/bluebottle,onepercentclub/bluebottle,onepercentclub/bluebottle,jfterpstra/bluebottle,onepercentclub/bluebottle