fix invalid yaml in conda environment.yml

[bjmuld]

Fixes #283

• [x] Tests pass
• [x] Appropriate changes to README are included in PR

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[bjmuld]

@googlebot I signed it!

[bjmuld]

Anything else needed? It might be the case that the environment.yml file had the (required) leading whitespace removed by some other tool... in which case, this fix might get clobbered again. But obviously, I have no knowledge of upstream processes.

Anything else needed, let me know!