update optimizely script (#292)

public/index.html

@@ -5,7 +5,11 @@
       <meta charset="utf-8">
       <meta name="viewport" content="width=device-width, initial-scale=1.0">
       <link rel="shortcut icon" href="<%=htmlWebpackPlugin.options.FAVICON_URL%>" type="image/x-icon" />
-      <% if (process.env.OPTIMIZELY_PROJECT_ID) { %>
+      <% if (process.env.OPTIMIZELY_URL) { %>
+        <script
+          src="<%= process.env.OPTIMIZELY_URL %>"
+        ></script>
+      <% } else if (process.env.OPTIMIZELY_PROJECT_ID) { %>
         <script
           src="<%= process.env.MARKETING_SITE_BASE_URL %>/optimizelyjs/<%= process.env.OPTIMIZELY_PROJECT_ID %>.js"
         ></script>