Specify channels and versions in Conda env ymls

[WenjieDu]

As @MaciejSkrabski suggested, building a new Conda environment for PyPOTS from current config files takes too much time. One possible solution is to explicitly specify the channels and versions of dependencies to shorten Conda's parsing process. @MaciejSkrabski tried in PR #7, but commits didn't pass unit tests (all failed due to conflict dependencies). Many thanks to Maciej's efforts anyway. And this idea deserves another try, out of question.

Additionally, PyPOTS should be released on Conda in the near future (so far, we can only install it from PyPI or source code).

[WenjieDu]

We have new progress in PR #12. Specifying channels can help reduce the whole test running time from 30~50mins to around 20~25mins, mostly cutting the time spent on Conda dependency parsing.