Athens: Update Athens CI for Go version 1.11

[komuw]

Go version 1.11 is scheduled to come out some time this month(August 2018) as per Go's regular release schedule[1]

There are already some PR's that have been put out in preparation for that, eg [2][3]
However, we need to update the CI pipeline[4] to use Go version 1.11 and also drop the use of dep for package management and instead use Go modules[5]

Using Go modules and Travis-CI[6]

ref:

1. https://github.com/golang/go/wiki/Go-Release-Cycle
2. https://github.com/gomods/athens/pull/371
3. https://github.com/gomods/athens/pull/378
4. https://github.com/gomods/athens/blob/master/.travis.yml
5. https://tip.golang.org/cmd/go/#hdr-Modules__module_versions__and_more
6. https://dave.cheney.net/2018/07/16/using-go-modules-with-travis-ci

[komuw]

I'll work on this.

I'm working on the assumption that everyone is okay if we drop dep in favour of Go modules for dependency management once Go version 1.11 comes out.

[arschles]

@komuw is this done now?

[komuw]

@arschles yes it is.

closed by; https://github.com/gomods/athens/pull/563