Add Python 3.7 to travis.yml

[mrocklin]

• Tests added / passed
• Passes flake8 dask

[mrocklin]

@TomAugspurger what is the right behavior for this:

  /home/travis/build/dask/dask/dask/dataframe/methods.py:333: FutureWarning: Sorting because non-concatenation axis is not aligned. A future version
  of pandas will change to not sort by default.
  
  To accept the future behavior, pass 'sort=False'.
  
  To retain the current behavior and silence the warning, pass 'sort=True'.
  
    out = pd.concat(dfs3, join=join)

[TomAugspurger]

For end-users doing something like

a = pd.DataFrame(columns=['a', 'b'])
b = pd.DataFrame(columns=['b', 'a'])
dd.concat([a, b])

we want that warning to come through, to match pandas.

Internal to dask.dataframe, we should ensure that we either have aligned columns, or specify sort if we rely on the output order. I can try to fix all these warnings this afternoon.

[mrocklin]

Does anyone mind me dropping this CI entry as we add one for 3.7? cc @jakirkham @jcrist

[jakirkham]

I do. We generally try to support both Python 2/3 in our codebase as there are inevitably scientific libraries that we want to pull in that haven't made the switch. Though I think this has gotten better in the past year or two, it's still a surprisingly common problem (even with new codes).

[mrocklin]

There is still another 2.7 entry. We used to have two. You might want to check the full file in this branch

[mrocklin]

@jakirkham does your concern still hold, given that there is another Python 2.7 entry?

[jakirkham]

Oops. Sorry. Missed the other one. This is fine.

[mrocklin]

OK, merging this in.