Add a cache for our node_modules on Travis
Add a cache for our node_modules on Travis

jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab