Drop support for Python 3.5

[leouieda]

We'll no longer build packages or test in 3.5.
Update the required python version in setup.py
to avoid pip installing a broken version.

Fixes #47

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

@fatiando/pooch if you're OK with this, I'll merge it in. I see that MetPy already dropped 3.5

[leouieda]

Moving ahead with this since no alarms went off.