Does not work on Linux due to case senstivity

[tdaede]

node_modules/node-firefox-find-app/index.js:var Promise = require('es6-Promise').Promise;
node_modules/node-firefox-stop-app/index.js:var Promise = require('es6-Promise').Promise;
node_modules/node-firefox-install-app/index.js:var Promise = require('es6-Promise').Promise;
node_modules/node-firefox-launch-app/index.js:var Promise = require('es6-Promise').Promise;

es6-promise is all lowercase in npm. Figured I'd file here than 4 times on the other projects.

[fwenzel]

Thank you!