docs: update supported python versions

[jlhood]

Also updates Travis build to test against Python 3.7.

Issue #, if available: #945

Description of changes: Update development guide's list of supported versions. Add python 3.7 to Travis build.

Description of how you validated changes: Travis build against this PR should validate the changes.

Checklist:

• Write/update tests
• make pr passes
• Update documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #968 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #968   +/-   ##
=======================================
  Coverage     94.8%   94.8%           
=======================================
  Files           69      69           
  Lines         3079    3079           
  Branches       580     580           
=======================================
  Hits          2919    2919           
  Misses          84      84           
  Partials        76      76

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[keetonian]

Add it to tox too: https://github.com/awslabs/serverless-application-model/blob/develop/tox.ini