freeze Go build dependencies in Gopkg.lock

* use pseudo-depdencies in build/build.go to convince dep
* update Travis, Dockerfile and Docs
* build.Dockerfile image now contains the Go build dependencies
* => faster builds
* bump pdu file after protoc update

fixes #106

build/build.go

@@ -0,0 +1,22 @@
+//+build windows
+
+// This package cannot actually be built, and since the Travis
+// tests run go test ./..., we have to avoid a build attempt.
+// Windows is not supported atm, so this works ¯\_(ツ)_/¯
+
+// This package is a pseudo-user of build-time dependencies for zrepl,
+// mostly for various code-generation tools.
+//
+// The imports are necessary to satisfy go dep
+package main
+
+import (
+	_ "fmt"
+	_ "github.com/alvaroloes/enumer"
+	_ "github.com/golang/protobuf/protoc-gen-go"
+	_ "golang.org/x/tools/cmd/stringer"
+)
+
+func main() {
+	fmt.Println("just a placeholder")
+}