Install a specific version of mime in node example

Since version 2 isn't compatible with the example code

Issue marijnh#228

Author: marijnh

20_node.txt

@@ -746,7 +746,7 @@ where the server script lives, you'll be able to use `require("mime")` to
 get access to the library:
 
 ----
-$ npm install mime
+$ npm install mime@1.4.0
 npm http GET https://registry.npmjs.org/mime
 npm http 304 https://registry.npmjs.org/mime
 [email protected] node_modules/mime