Remove pytest-xdist and fix coverage

[jayvdb]

And is the minimal fixes needed to make the tree green again, which is failing because of xdist and six
https://travis-ci.org/coala/coala-quickstart/builds/

This depends on https://gitlab.com/coala/mobans/merge_requests/128

It also solves part of #295

[TravisBuddy]

Travis tests have failed

Hey @jayvdb,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

1st Build

View build log

pytest

requests-mock~=1.2
 ipdb~=0.11
 pip<10
-six>=1.11.0
 wheel~=0.29
The command ".ci/check_moban.sh" exited with 1.

store build cache
changes detected (content changed, file is created, or file is deleted):\n/home/travis/.cache/pip/http/0/4/e/0/1/04e017851dbfe81db92d092eaaf2526622c3950bcb8f2696e57143da
/home/travis/.cache/pip/http/0/7/1/3/2/07132491788c92632f9eab153a9d5930f26718f20db78c7a10f7279c
/home/travis/.cache/pip/http/1/1/3/7/f/1137f1fd0be3aa5e638d02149f2046f2b6992713042fe73d7ecc3f4b
/home/travis/.cache/pip/http/1/5/8/9/3/1589337688253359c9ce9ec56514af56c06992df704c6ca057f86e56
/home/travis/.cache/pip/http/1/8/0/c/9/180c97d0ce330d4b5bdf9a3714532b2cb5ff73126efb503833a43e4b
/home/travis/.cache/pip/http/3/3/a/d/7/33ad7fe31489f80e15fea42300adbfc4d75c0efc76d75cfab7d2e5b0
/home/travis/.cache/pip/http/4/5/9/7/6/45976c194ebb9180bf7410d422bcad653ce13bcb64dda0d07d72b09d
/home/travis/.cache/pip/http/4/9/5/1/d/4951d5b1cc8cf21ac6def23284ec38188e32c306e8f7d8cf3c44264d
/home/travis/.cache/pip/http/4/9/f/5/b/49f5b6df1993e2e89e871ce7123446e622bbf71141094da3530e6108
/home/travis/.cache/pip/http/8/0/7/8/0/80780a36033f35b0aaa2e31728fae9cc8e5e456217a81645ce80839e
/home/travis/.cache/pip/http/8/5/e/d/c/8\n...
changes detected, packing new archive
uploading PR.333/cache-linux-xenial-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--python-3.6.tgz
cache uploaded


Done. Your build exited with 1.

TravisBuddy Request Identifier: 50b377a0-9301-11e9-bc3e-3d3ede91b564