Use dep for vendor

Like we did for circleci/build-agent#678

Gopkg.toml

@@ -0,0 +1,42 @@
+# Gopkg.toml example
+#
+# Refer to https://github.com/golang/dep/blob/master/docs/Gopkg.toml.md
+# for detailed Gopkg.toml documentation.
+#
+# required = ["github.com/user/thing/cmd/thing"]
+# ignored = ["github.com/user/project/pkgX", "bitbucket.org/user/project/pkgA/pkgY"]
+#
+# [[constraint]]
+#   name = "github.com/user/project"
+#   version = "1.0.0"
+#
+# [[constraint]]
+#   name = "github.com/user/project2"
+#   branch = "dev"
+#   source = "github.com/myfork/project2"
+#
+# [[override]]
+#   name = "github.com/x/y"
+#   version = "2.4.0"
+#
+# [prune]
+#   non-go = false
+#   go-tests = true
+#   unused-packages = true
+
+
+[[constraint]]
+  name = "github.com/machinebox/graphql"
+  branch = "master"
+
+[[constraint]]
+  name = "github.com/spf13/cobra"
+  version = "0.0.3"
+
+[[constraint]]
+  name = "github.com/spf13/viper"
+  version = "1.0.2"
+
+[prune]
+  go-tests = true
+  unused-packages = true

README.md

@@ -4,7 +4,7 @@ Summary
 
 ## Requirements
 
-* Go 1.x
+* Go 1.9+
 * Make
 * ...
 
@@ -17,13 +17,30 @@ It's written in Go. If you are new to Go, we recommend the following resources:
 
 ### 1. Go Dependencies
 
+Install `dep`:
+https://github.com/golang/dep
+
+On MacOS:
+
+```
+$ brew install dep
+$ brew upgrade dep
+```
+
+On Linux, etc:
+
 ```
-$ go get github.com/spf13/cobra
-$ go get github.com/spf13/viper
-$ go get github.com/machinebox/graphql
-$ go get ...
+$ curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
 ```
 
+Ensure dependencies are installed:
+
+```
+$ dep ensure
+```
+
+TODO: we want `make` to check vendor? They are committed...
+
 ### 2. Build it
 
 ```