!!! is deprecated, you must now use doctype

[ManasJayanth]

As seen on this SO question and when I ran the code from ch05 I came across the following,
Error: /Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/views/index.jade:1
> 1| !!! 5
2| html(lang="en")
3| head
4| title Social Network

`!!!` is deprecated, you must now use `doctype`
    at Object.Lexer.doctype (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/lexer.js:248:13)
    at Object.Lexer.next (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/lexer.js:833:15)
    at Object.Lexer.lookahead (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/lexer.js:113:46)
    at Parser.lookahead (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/parser.js:111:23)
    at Parser.peek (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/parser.js:88:17)
    at Parser.parse (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/parser.js:126:26)
    at parse (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/jade.js:95:62)
    at Object.exports.compile (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/jade.js:152:9)
    at Object.exports.render (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/jade.js:256:15)
    at Object.exports.renderFile (/Users/prometheansacrifice/development/github-repos/book-node-mongodb-backbone/ch05/node_modules/jade/lib/jade.js:293:18)

The SO question directed me to this leading me to suggest to you to update all the jade code accordingly