ci: Add support for go 1.15.x to .travis.yml

[pims]

Also drops testing against go 1.12.x

It might be worth adding support for Github Actions down the road to reduce the chance of a missed "check" on PRs in the future.

For now, testing with a recent go version would be nice.

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[pims]

@googlebot I signed it!

[alandonovan]

Thanks!