Fix octals for strict mode

[alarner]

When trying to use keypress in strict mode node complains because of use of octal literals. This PR changes octal literals to a format that works in strict mode.

node --use_strict test.js
/Users/alarner/oss/keypress/index.js:375
    key.y = s.charCodeAt(5) - 040;
                              ^^^

SyntaxError: Octal literals are not allowed in strict mode.
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/alarner/oss/keypress/test.js:2:16)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)

[hellais]

@TooTallNate do you think you could merge this PR and push a new package to npm? I think this is a good fix and it would be useful to have.