Cache node_modules directory on Travis
Cache node_modules directory on Travis

Npm@5 does a better job at bringing the node_modules directory into
shape so we can safely cache this directory.

thecoolestguy/frontend,djvoa12/frontend,dartajax/frontend,ilios/frontend,jrjohnson/frontend,jrjohnson/frontend,gabycampagna/frontend,gboushey/frontend,gabycampagna/frontend,gboushey/frontend,dartajax/frontend,djvoa12/frontend,thecoolestguy/frontend,ilios/frontend