dep: require tools to be vendored

This change ot Gopkg.toml will make it easier to build the linting and
formatting tools. Vendoring their source also makes sure that updates to
these tools do not break the build.

Gopkg.toml

@@ -1,3 +1,12 @@
+# Tools required for generating, liniting, and formatting code
+required = [
+  "github.com/golang/lint/golint",
+  "github.com/golang/protobuf/protoc-gen-go",
+  "golang.org/x/tools/cmd/goimports",
+  "honnef.co/go/tools/cmd/megacheck",
+  "github.com/wadey/gocovmerge"
+]
+
 # Source dependancies
 [[constraint]]
   name = "github.com/golang/protobuf"