Error thrown if optional dependency safe-json-stringify is not installed

[galenandrew]

Loading the optional dependency safe-json-stringify is not properly handled when it is not installed which results in an error being thrown (see stack trace below).

Error: Cannot find module 'safe-json-stringify'
    at Function.Module._resolveFilename (module.js:489:15)
    at Function.Module._load (module.js:439:25)
    at Module.require (module.js:517:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/app/node_modules/bunyan-cloudwatch/index.js:5:25)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)

PR on its way.