Add our package dependencies

This repo was unbuildable without knowing the versions of the packages
it was originally coded against. Now that golang has some semi-sane
dependency management we can specify those. Prior to building we can
call 'dep ensure' to get the proper versions in the vendor directory.

This thing should really just be a script that calls out to git and
buildkite-agent, but that's a discussion for the next time we need to
mess with it.

Gopkg.toml

@@ -0,0 +1,38 @@
+
+# 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"
+
+
+[[constraint]]
+  branch = "2-1-stable"
+  name = "github.com/buildkite/agent"
+
+[[constraint]]
+  name = "github.com/hashicorp/hil"
+  revision = "3eb5226cd1c40e1788b3295b5bafb5a2fcac90a6"
+
+[[constraint]]
+  branch = "v24"
+  name = "gopkg.in/libgit2/git2go.v24"
+
+[[constraint]]
+  branch = "v2"
+  name = "gopkg.in/yaml.v2"