Bump notebook from 5.7.4 to 5.7.8

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 5.7.4 to 5.7.8.
- [Release notes](https://github.com/jupyter/jupyterhub/releases)
- [Changelog](https://github.com/jupyterhub/jupyterhub/blob/master/CHECKLIST-Release.md)
- [Commits](https://github.com/jupyter/jupyterhub/commits)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Diff for: 2019-08-06-bob-ross-requirements.txt

@@ -30,7 +30,7 @@ mistune==0.8.4
 mizani==0.5.4
 nbconvert==5.5.0
 nbformat==4.4.0
-notebook==6.0.0
+notebook==5.7.8
 numpy==1.17.0
 palettable==3.2.0
 pandas==0.25.0

Diff for: requirements.txt

@@ -34,7 +34,7 @@ more-itertools==6.0.0
 nbconvert==5.4.0
 nbformat==4.4.0
 nltk==3.4
-notebook==5.7.4
+notebook==5.7.8
 numpy==1.16.0
 palettable==3.1.1
 pandas==0.24.1