Revert "Add npm cache"
Revert "Add npm cache"

This reverts commit f9ecdcb480badc21facbea17bc0f71ed216fcdda.

Travis is unhappy about adding an NPM cache as it can't find
matrix-js-sdk for some reason

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk