change type when reading the diasend xls to get past  error

With this change I'm able to import from diasend and view both the diasend and dexcom data in my locally running blip instance.

Before making the change I got this error:
> /Users/jcalabre/Development/tidepool/jellyfish/node_modules/tidepool-animas-diasend-data/node_modules/xlsjs/xls.js:5538
> 		case 'binary': return f.charCodeAt(0);
> 		                        ^
> TypeError: Object ��ࡱ�

The encoding mess after Object was printed to the console