travis: don't cache pip
travis: don't cache pip

We currently have so few dependencies that it is not really relevant
anymore.

Signed-off-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>

efiop/dvc,efiop/dvc,dmpetrov/dataversioncontrol,dataversioncontrol/dvc,dmpetrov/dataversioncontrol,dataversioncontrol/dvc