Bug 1390517 - Travis: Make the selenium job cache node_modules too
Bug 1390517 - Travis: Make the selenium job cache node_modules too

Previously the selenium job was spending 20 seconds in `yarn install`.

tojonmz/treeherder,tojonmz/treeherder,edmorley/treeherder,edmorley/treeherder,KWierso/treeherder,tojon/treeherder,tojon/treeherder,KWierso/treeherder,tojonmz/treeherder,edmorley/treeherder,edmorley/treeherder,tojon/treeherder,tojon/treeherder,tojonmz/treeherder,tojonmz/treeherder,KWierso/treeherder,KWierso/treeherder,tojonmz/treeherder