depend on existing yarn:install rake task (#43)

Author: mweitzel

lib/tasks/jsbundling/build.rake

@@ -1,7 +1,7 @@
 namespace :javascript do
   desc "Build your JavaScript bundle"
   task :build do
-    unless system "yarn install && yarn build"
+    unless system "yarn build"
       raise "jsbundling-rails: Command build failed, ensure yarn is installed and `yarn build` runs without errors"
     end
   end
@@ -14,3 +14,7 @@ if Rake::Task.task_defined?("test:prepare")
 elsif Rake::Task.task_defined?("db:test:prepare")
   Rake::Task["db:test:prepare"].enhance(["javascript:build"])
 end
+
+if Rake::Task.task_defined?("yarn:install")
+  Rake::Task["javascript:build"].enhance(["yarn:install"])
+end