try to fix tests

[Carreau]

Json schema pre seem to require attrs>17.3.0.

So pin it in tests.

Should solve issues of people like @Hyaxia in #4038

Let's see if the test pass only with this change.

[Hyaxia]

Looks like travis gives the same errors as the one I'm having at #4038

[Carreau]

Looks like travis gives the same errors as the one I'm having at #4038

Yeah I pinned to >=17.3 instead of >=17.4. I've pushed changes.

[Hyaxia]

Hey, it looks like the tests are doing problems again.
#4036 I commited few other changes and the tests now fail in appveyor.
Fail at the package installation part, at pip install .[test].

It looks like the problem is something with the setuptools or the pip itself,
I have tried to play a little bit with the configuration for the appveyor and see if it works if i downgrade
the different installations but it doesn't seem to work.

getting error pip._vendor.packaging.requirements.InvalidRequirement: Invalid requirement, parse error at "'; extra '"

[jph00]

FYI it appears that this bug is breaking all environments that are using jupyter - for instance: http://forums.fast.ai/t/conda-env-update-error-on-paperspace/23483

If this is the source of this recent issue, perhaps you could revert the recent change that caused it until it's fixed?

[Hyaxia]

@jph00 see my temporary fix for the bug 4c1d62f
It seems like there is a problem with the current of nbconvert or its dependencies that
anaconda installs, so we just use pip instead.

[jph00]

Thanks for the tip!

[blink1073]

Thanks @Carreau, forging ahead!