Add boto to install_requires since it does use it for s3 node

[yarikoptic]

Found by @effigies who ate all 2 sausages!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.56%. Comparing base (e4d1050) to head (3c478c4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #138 +/- ## ======================================= Coverage 77.56% 77.56% ======================================= Files 60 60 Lines 4720 4720 Branches 956 956 ======================================= Hits 3661 3661 Misses 895 895 Partials 164 164 ```

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