Convert to Go modules (#1178)

* Convert to Go modules

* Update promu config.
* Convert to Go modules.
* Update vendoring.
* Update Makefile.common.
* Update circleci config.
* Use Prometheus release tar for promtool.
* Fixup unpack

* Use temp dir for unpacking tools.
* Use BSD compatible tar command.
* OpenBSD mkdir doesn't support `-v`.

Signed-off-by: Ben Kochie <superq@gmail.com>

vendor/github.com/beevik/ntp/.travis.yml

@@ -0,0 +1,15 @@
+language: go
+sudo: false
+
+go:
+  - 1.7.x
+  - 1.8.x
+  - 1.9.x
+  - tip
+
+matrix:
+  allow_failures:
+    - go: tip
+
+script:
+  - go test -v ./...