Enforce version constraint and clarify ubuntu install

[austinbyers]

to: @chunyong-lin cc: @airbnb/binaryalert-maintainers size: small resolves: #108

Background

There have been multiple issues opened about failing unit tests because Python 3.5 is the default for some systems (like Ubuntu), but BinaryAlert requires Python 3.6 (for type annotations). #41 clarified the Python version in the documentation, but we need more.

Changes

• The CLI now fails if it is run with Python < 3.6
• The documentation includes a warning that 3.5 is not supported
• Expanded on the documentation for installing BinaryAlert in Ubuntu

Testing

• Installed BinaryAlert on a fresh Ubuntu instance, following this updated documentation

[coveralls]

Coverage decreased (-0.1%) to 93.017% when pulling 608e0015e94aa346f58dbf439cf512acb56c74f0 on austin-ubuntu-docs into 775092d3db7b6464283b70c870112eb71517baca on master.