This switches atomic to Go modules. This has the effect of simplifying
the Makefile and the Travis build, as well as getting rid of the overly
complicated coverage script we copied here.
Tools dependencies (currently only golint) were added to the tools.go.
As a result of this change, we no longer support the non-vanity import path
github.com/uber-go/atomic. Users must use the vanity import path, or add a
`replace` directive.
Supersedes #40
