Parallelize travis tests within a single vm

[timokau]

Description

Let pytest-xdist handle the parallelization instead of manually
specifying a split. Should run on at least two cores:
https://docs.travis-ci.com/user/reference/overview/#virtualisation-environment-vs-operating-system

This is an experiment to compare the runtime with https://github.com/kiudee/cs-ranking/pull/64/files.

Motivation and Context

The tests are too slow and manually specifying a split as done in #64 is error-prone.

Does this close/impact existing issues?

#51, #64

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[coveralls]

Pull Request Test Coverage Report for Build 553

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+11.3%) to 55.381%

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Totals
Change from base Build 527:	11.3%
Covered Lines:	4153
Relevant Lines:	7499

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💛 - Coveralls