Simplify CI config

[hugovk]

• Test Python 3.8 beta, before it's October release

• sudo: is no longer needed on Travis CI: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

• Default to Trusty, explicitly use Xenial where needed

[asherf]

@hugovk python 3.8 was release... you can probably update the PR to use the GA version.

[codecov-io]

Codecov Report

Merging #1838 into develop will decrease coverage by 0.06%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##           develop    #1838      +/-   ##
===========================================
- Coverage    92.79%   92.73%   -0.07%     
===========================================
  Files           53       53              
  Lines        10332    10293      -39     
===========================================
- Hits          9588     9545      -43     
- Misses         744      748       +4

Impacted Files	Coverage Δ
botocore/awsrequest.py	97.56% <ø> (-0.81%)	⬇️
botocore/__init__.py	100% <100%> (ø)	⬆️
botocore/compat.py	96.4% <83.33%> (+2.28%)	⬆️
botocore/handlers.py	96.51% <0%> (-0.47%)	⬇️
botocore/hooks.py	95.98% <0%> (-0.41%)	⬇️
botocore/credentials.py	98.53% <0%> (-0.34%)	⬇️
botocore/utils.py	98.33% <0%> (-0.13%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 160e4c0...76499bc. Read the comment docs.

[hugovk]

Thanks, updated!

[hugovk]

Python 3.8 was added in #1922, so this PR now simply simplifies the CI config.

[hugovk]

Merge conflict resolved.