_js: improve build script

First, make it check that the 'go' command is the version we require.

Second, use 'go run' to execute gopherjs, which means the developer
doesn't need to run 'go install' first, nor pollute their $PATH with the
gopherjs version we need.

_js/build

@@ -1,13 +1,13 @@
 #!/bin/sh
 
-# If you get weird errors like 'undeclared name: CacheLinePadSize', try being
-# explicit about the Go 1.11.x GOROOT, like:
-#
-#     GOROOT=/usr/lib/go ./build
+if ! go version | grep go1.11; then
+	echo "Go 1.11.x required to build."
+	exit 1
+fi
 
 export GO111MODULE=on # build in module-aware mode
 
-gopherjs build -o index.js || exit 1
+go run github.com/gopherjs/gopherjs build -o index.js || exit 1
 
 node testmain.js || exit 1