Copying source code from TRAVIS_BUILD_DIR into gopath more idempotently

So that `mkdir -p` operations performed prior to this step don't
result in the source code being copied into a nested directory, as
is the case when using casher.
Merge pull request #301 from travis-ci/db-copy-go-src-better-er

Copying source code from TRAVIS_BUILD_DIR into gopath more idempotently