Drop unsupported versions and Add new versions

[hirokinko]

• Add support for Django 2.0 or higher
• Add support for Python 3.7
• Drop support for Django (1.10 or lower)
• Drop support for Python (2.6, 3.3)
• Use tox to run tests and env matrix.

[coveralls]

Coverage decreased (-5.4%) to 86.441% when pulling 679a6e4 on uncovertruth:drop_unsupported_versions into 94ff6e1 on goinnn:master.

[hirokinko]

@goinnn Does this pull request have any problem?

[benzkji]

would be great to have an up to date version of django-multiselectfield

[wolph]

As a temporary workaround you can install this git version through pip like this:

pip install 'git+https://github.com/uncovertruth/django-multiselectfield.git@drop_unsupported_versions#egg=django-multiselectfield' 

[karolyi]

@hirokinko the PR is failing because you added code that lowered code coverage. add more tests covering the new lines and your PR will not fail.

[tomasgarzon]

Thanks @hirokinko for this great contribution, could you add some additional test in order to increase the coverage? This PR is failing and we need to fix it before merging to master.