Factor build tool installation out into a script

For now it only moves protoc installation.  Support for version-pegging
“go get”-able tools is included, but since tools/tools.go lists no tools
yet, it is a no-op.  The list will be populated/migrated in later
commits.

.travis.yml

@@ -26,7 +26,7 @@ install:
   - go get github.com/harmony-ek/gencodec
   - go get github.com/golang/mock/mockgen
   - go get github.com/golang/protobuf/protoc-gen-go
-  - ./scripts/install_protoc.sh -V 3.6.1
+  - scripts/install_build_tools.sh
 script:
   - go build -v ./...
   - ./scripts/travis_checker.sh

scripts/install_build_tools.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -eu
+
+unset -v progdir
+case "${0}" in
+/*) progdir="/";;
+*/*) progdir="${0%/*}";;
+*) progdir=".";
+esac
+
+sed -n 's/^	_ "\([^"]*\)"$/\1/p' "${progdir}/../tools/tools.go" | \
+	xargs "${progdir}/goget.sh"
+
+"${progdir}/install_protoc.sh" -V 3.6.1