[Nit] Be clearer in CONTRIBUTING.md which tox ops are run with `tox`

[ajratner]

Just a nit: should be clearer e.g. that tox -e complex not run (any others?)

[henryre]

This is covered in https://github.com/snorkel-team/snorkel/blob/master/CONTRIBUTING.md#complexintegrationlong-running-tests, but we can also add indicators for the commands in the block here (https://github.com/snorkel-team/snorkel/blob/master/CONTRIBUTING.md#testing-and-committing) as to whether they're run with tox and/or by Travis. That work?

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[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.