Remove sudo from Travis template

[uilianries]

• sudo is no longer required to run Docker
• Update Python version to 3.7

Changelog: Fix: Remove sudo from Travis CI template
Docs: conan-io/docs#1270

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• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}