ci: fix build for go 1.13 (#96)

Updates modules handling so build passes in Go 1.13. Bypasses modules for tools install in bootstrap. Upgrades golangci-lint version to avoid bug with v1.17.1 under Go 1.13.

Fixes #95

script/bootstrap

@@ -1,17 +1,18 @@
 #!/bin/bash -ex
 
-# Limit dependencies to golang.org/x repos
-go get -u \
-    golang.org/x/arch/x86/... \
-    golang.org/x/tools/go/...
+# Install dependencies.
+go mod download
 
 # Standalone version of the asmdecl analysis tool.
 go install ./internal/cmd/asmdecl
 
 # Install golangci-lint
-golangci_lint_version='v1.17.1'
+golangci_lint_version='v1.18.0'
 curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin ${golangci_lint_version}
 
+# Turn off modules for tools install.
+export GO111MODULE=off
+
 # embedmd required for documentation generation
 go get -u github.com/campoy/embedmd