ci: do not use go modules

Attempt to fix a stream of CI failures that started on the addition of
Go modules.

I wonder if we are falling foul of this warning in the wiki
https://github.com/golang/go/wiki/Modules.

"The module cache in Go 1.11 can sometimes cause various errors,
primarily if there were previously network issues or multiple go
commands executing in parallel (see #26794, which is addressed for Go
1.12). As a troubleshooting step, you can copy $GOPATH/pkg/mod to a
backup directory (in case further investigation is warranted later), run
go clean -modcache, and then see whether the original problem persists."

Updates #21

.travis.yml

@@ -1,8 +1,6 @@
 language: go
 go:
 - 1.11.1
-env:
-- GO111MODULE=on
 install:
 - ./script/bootstrap
 script:

script/bootstrap

@@ -1,7 +1,9 @@
 #!/bin/bash -ex
 
-# Install go module dependencies.
-go mod download
+# Limit dependencies to golang.org/x repos
+go get -u \
+    golang.org/x/arch/x86/... \
+    golang.org/x/tools/go/...
 
 # Install golangci-lint
 golangci_lint_version='v1.12.3'