[issue36307] Upgrade Travis CI config to Xenial from near-EoL Trusty and remove obsolete sudo: false key

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

https://travis-ci.org/python/cpython/jobs/507821992
"pyenv: python3.7: command not found"

Hmm, `dist: xenial; group beta` seems not stable yet.
I'll try removing "group: beta" for stable build.

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<https://bugs.python.org/issue36307>
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