Revert "travis: don't -y on build-depends"
Revert "travis: don't -y on build-depends"

This reverts commit 17f8db234acc85dfd4e0cd9fcf9c18d1e673bc04.

danielhkl/matplotlib2tikz,nschloe/matplotlib2tikz,dougnd/matplotlib2tikz,m-rossi/matplotlib2tikz