Add legacy support for < 2.0.0 build option

[Syntaf]

There are a number of repos counting on the old source option and I'm not a fan of creating breaking changes (even when it's a major version upgrade honestly). I think it would be helpful to support --source coming before build for the sake of older repos that are now going to be failing upon build.

One idea is that we could keep the current source parameter, and add another parameter to travis-sphinx that is also called source.

• If --source is used for both build and travis-sphinx, then we error out and tell the user to use the one after build
• If --source is used before build, we provide a warning message that this use of source is deprecated and should not be favored.

[lmignon]

@Syntaf I've added pytest to be able to enforce existing rules and avoid future regression see #29
In the same time we are now sure that travis-spinx is compliant with py2 and py3.
It's possible to support old version by modifying a little bit the code and to output a deprecation message. I can do it in the next days.

[lmignon]

@Syntaf After some tries, it appears that supporting a legacy mode is not possible without disabling checks natively done by the click library... IMO it's a bad idea.