Bump tensorflow in /machine-learning/nlp/spam-classifier

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.4 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.4...v2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

machine-learning/nlp/spam-classifier/requirements.txt

@@ -3,4 +3,4 @@ keras
 tqdm
 numpy
 keras_metrics
-tensorflow==1.15.4
+tensorflow==2.4.0