text: Title: Issue while running the colab notebook on ploomber examples
  Body: ```
  File "/usr/local/lib/python3.7/dist-packages/ploomber/__init__.py", line 2, in <module>
      from ploomber.dag.dag import DAG
    File "/usr/local/lib/python3.7/dist-packages/ploomber/dag/dag.py", line 93, in <module>
      from ploomber.dag.dagclients import DAGClients
    File "/usr/local/lib/python3.7/dist-packages/ploomber/dag/dagclients.py", line 4, in <module>
      from ploomber.tasks.abc import Task
    File "/usr/local/lib/python3.7/dist-packages/ploomber/tasks/__init__.py", line 6, in <module>
      from ploomber.tasks.notebook import NotebookRunner, ScriptRunner
    File "/usr/local/lib/python3.7/dist-packages/ploomber/tasks/notebook.py", line 15, in <module>
      from nbconvert.exporters.webpdf import WebPDFExporter
  ModuleNotFoundError: No module named 'nbconvert.exporters.webpdf'
  [Errno 2] No such file or directory: 'first-pipeline'
  /content
  total 4
  drwxr-xr-x 1 root root 4096 May  3 13:42 sample_data
  ```
  
  ploomber, version 0.18.1
  