travis: run goveralls inside the docker container

Factor code and reorganize location of go cache and go paths such
that subsequent invocations reuse both dirs.

.travis.yml

@@ -3,12 +3,6 @@ dist: xenial
 language: minimal
 services:
 - docker
-env:
-- OS_NAME: linux
-before_script:
-- go get golang.org/x/tools/cmd/cover
-- go get github.com/mattn/goveralls
 script:
 - ./build.sh _travis-${TRAVIS_OS_NAME}
-- $GOPATH/bin/goveralls -coverprofile=coverage.cov -service=travis-ci
 - ./scripts/travis_test.sh