* TeamCity: change test matrix and test scripts

Changes test matrix to test:

* 1.13 through 1.16 on linux/amd64
* 1.15, 1.16 on windows/amd64
* 1.15 on linux/386

Also includes tests on go-tip for windows/amd64 and linux/amd64
(currently disabled, but can be turned on in a PR).

Changes test scripts to automatically select the latest patch version
of Go.

* CI: remove Travis-CI and Appveyor configuration

Fixes #2134

See Issue #2134

Thank you to:  @polinasok, @chainhelen, @alexsaezm, @bhcleek,
@letientai299, @gadelkareem and @b00f

Instead of selectively excluding this directory, hide it from the go
tooling by applying the "_" prefix.

Autogenerates .travis.yml configuration using the contents of
pkg/goversion/compat.go.
Also:
- excludes testing of windows/amd64/tip and linux/arm64/tip (Travis-CI
  can't get Go at tip for those combinations).
- Removes Go 1.11 from pkg/goversion/compat.go which we don't test
  anymore.