Move Travis CI make task into a bash script

Move the make task for Travis CI to a bash script. This is in
preparation for removing the Makefile.

Also, simplify the command-line options for `go test`; `-test.v` has the
same behaviour as the shorter `-v` and `-run` is removed as all tests
are run by default so this was unnecessary.
Remove Makefile

Remove the Makefile because:

  - it removes a dependency on `make(1)`
  - a lot of the tasks, e.g. `vagrant-up`, add little value
  - the `RUN` variable unnecessarily adds the `.+` regex; the default is
    to run all tests anyway
  - as we add more flags (i.e. vendor-specific flags), using a Makefile
    obscures these options
Skip 'install' step

Tell Travis CI to run `true(1)` for the `install` step.

This is to prevent Travis from running `go get -v ./...`, which fails[1]
with:

    ./helpers.go:117: undefined: edgeHost
    ./helpers.go:146: undefined: client
    ./helpers.go:147: undefined: requestSlowThreshold
    ./helpers.go:150: undefined: debugResp
    ./helpers.go:215: undefined: client
    ./helpers.go:276: undefined: edgeHost
    ./helpers.go:279: undefined: originServer
    ./helpers.go:346: undefined: originServer

I believe this is due to the way we're using flags in a suite of tests
(running `go test` works fine).

We don't need to run `go get` as we don't have any package dependencies
outside of Go's standard library. By skipping Travis' `install` step, we
prevent it from running `go get`.

Note that it that this wasn't a problem before because Travis apparently
doesn't run `go get` if it detects a Makefile[2], which is consistent
with the documentation here:
http://docs.travis-ci.com/user/languages/go/#Default-Test-Script

[1]: https://travis-ci.org/alphagov/cdn-acceptance-tests/builds/33156531
[2]: https://travis-ci.org/alphagov/cdn-acceptance-tests/builds/33047132
Rename common_setup_test.go to main_test.go

Which better follows Go convention.
Merge pull request #93 from alphagov/75804242-consolidate-docs

[#75804242] Consolidate documentation on how to run the tests