Update how dependencies are created

[ljvmiranda921]

Now, we're using pip-tools as a way to manage all our dependencies. We need to set it up first using an .in file, then compile it via make requirements.txt or make requirements-dev.txt. In this manner, all dependencies for the project are pinned explicitly.

Reference: #262

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 read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[ljvmiranda921]

matplotlib==3.0.2, the version we actually need, cannot be installed in py34. Let's remove support for that!