Travis: simplify build by not using pip cache, nor upgrade it
Travis: simplify build by not using pip cache, nor upgrade it

It is only used for the vint build and not necessary really.

Closes https://github.com/neomake/neomake/pull/1103.

bonds/neomake,nerdrew/neomake,euclio/neomake,euclio/neomake,neomake/neomake,nerdrew/neomake,codeinabox/neomake,bonds/neomake,neomake/neomake