title: `jupyterlab` doesn't recognize `nodejs` and `npm` installations
body: <p>I'm having troubles installing the <code>jupyterlab_bokeh</code> extension.</p>
  
  <pre><code>$ jupyter labextension install jupyterlab_bokeh
  Errored, use --debug for full output:
  ValueError: Please install nodejs and npm before continuing installation. nodejs may be installed using conda or directly from the nodejs website.
  </code></pre>
  
  <p>I'm using a virtual environment (<code>pyenv</code> plus <code>pyenv-virtualenvwrapper</code>). I've tried to install the two packages first on my local OS (Ubuntu 16.04) and reinstalled <code>jupyterlab</code> but that didn't help. I've also tried to install both <code>nodejs</code> and <code>npm</code> via <code>pip</code> on the virtual enviroment, after having uninstalled the packages on my OS just to make sure they were not conflicting. Unfortunately, neither of the tentative succeeded.</p>
  
  <p>The weird thing is that if I type the nodejs command on my terminal, I can access a <code>nodejs</code> REPL.</p>
  
  <pre><code>bokeh==0.12.13
  jupyter-client==5.2.1
  jupyter-core==4.4.0
  jupyterlab==0.31.1
  jupyterlab-launcher==0.10.2
  </code></pre>
  
  <p>The problem is not limited to the extension <code>jupyterlab_bokeh</code>. I can replicate the same error with the extension <code>jupyterlab-manager</code>.</p>
  
  <p>I've actually noticed that if I run jupyter <code>labextension list</code> I can see both extensions as correctly installed:</p>
  
  <pre><code>$ jupyter labextension list
  JupyterLab v0.31.1
  Known labextensions:
     app dir: /home/gianluca/.virtualenvs/rethinking/share/jupyter/lab
  @jupyter-widgets/jupyterlab-manager
          @jupyter-widgets/jupyterlab-manager v0.33.0  enabled  OK
  jupyterlab_bokeh
          jupyterlab_bokeh v0.2.0  enabled  OK
  
  Build recommended, please run `jupyter lab build`:
      @jupyter-widgets/jupyterlab-manager needs to be included in build
      jupyterlab_bokeh needs to be included in build
  </code></pre>
  
  <p>However, as you can read in the above snipped, a built is recommended. If I try to run the build, this is the error message I see:</p>
  
  <pre><code>$ jupyter lab build
  [LabBuildApp] &gt; node /home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/staging/yarn.js install
  Traceback (most recent call last):
    File "/home/gianluca/.virtualenvs/rethinking/bin/jupyter-lab", line 11, in &lt;module&gt;
      sys.exit(main())
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyter_core/application.py", line 266, in launch_instance
      return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/traitlets/config/application.py", line 658, in launch_instance
      app.start()
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/notebook/notebookapp.py", line 1431, in start
      super(NotebookApp, self).start()
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyter_core/application.py", line 255, in start
      self.subapp.start()
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/labapp.py", line 64, in start
      command=command, logger=self.log)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/commands.py", line 238, in build
      command=command, clean_staging=clean_staging)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/commands.py", line 376, in build
      self._run(['node', YARN_PATH, 'install'], cwd=staging)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/commands.py", line 1158, in _run
      proc = Process(cmd, **kwargs)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/process.py", line 72, in __init__
      self.proc = self._create_process(cwd=cwd, env=env)
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/process.py", line 130, in _create_process
      cmd[0] = which(cmd[0], kwargs.get('env'))
    File "/home/gianluca/.virtualenvs/rethinking/lib/python3.6/site-packages/jupyterlab/jlpmapp.py", line 59, in which
      raise ValueError(msg)
  ValueError: Please install nodejs and npm before continuing installation. nodejs may be installed using conda or directly from the nodejs website.
  </code></pre>
  
  <p>Again, both <code>nodejs</code> and <code>npm</code> are installed in the operating system.</p>