Remove m2r as dependency

[lwasser]

1. change code_of_conduct to .rst
2. remove mr2 in the deps

https://github.com/earthlab/matplotcheck/blob/master/dev-requirements.txt#L5

[lwasser]

@nkorinek can you add the PR where this was addressed so i can close this issue? i'm trying to close issues that have been addressed!!

[nkorinek]

#247 pr here!

[lwasser]

thank you !! closing!!