Refactored serialport adaptor to work with latest version of serialport

[charliegerard]

In the latest version of node-serialport, doing require('serialport').SerialPort is deprecated so Sphero.js does not work anymore.
Instead, serialport should be required with only require('serialport') so I simply refactored the serialport.js file and it seems to be working fine now.

[mefellows]

The issue here seems to be that serialport is not declared as a dependency in the package, but is installed only in the travis build. It works for you @charliegerard because, presumably, you've installed a later version locally.

Is there a reason as to why it's not in the package as a dependency?

Anywho, thanks for the fix!

[dariobraun]

Just stumbled upon this issue. You might wanna use this PR to fix this!