Remove exists-sync dependency

[Bartheleway]

This should close #5431

[runspired]

Thank you for the contribution! I'm not sure why ember beta and canary are broken. Trying to get that resolved so this will merge :)

[runspired]

I checked into it locally and those versions of Ember fail because of this regression: emberjs/ember.js#16511

Merging the PR, thanks :)

[alex-ushakov-93]

Error creating new application. Removing generated directory `./ember-quickstart`
Command failed: npm install --loglevel error
npm WARN deprecated exists-sync@0.0.4: Please replace with usage of fs.existsSync
npm WARN deprecated exists-sync@0.0.3: Please replace with usage of fs.existsSync
npm ERR! code ERR_STREAM_WRITE_AFTER_END
npm ERR! write after end

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/alex/.npm/_logs/2018-04-13T19_20_50_816Z-debug.log

Stack Trace and Error Report: /tmp/error.dump.db4df0879d38eb40d7c085354ee5eb52.log

alex@acer:~/Desktop$ yarn info ember-cli | grep version
  versions: 
  version: '3.1.1',
alex@acer:~/Desktop$ 

?

[runspired]

@alex-ushakov-93 This just removes a deprecation warning, it isn't an error. It looks like something else went wrong.