Update logo's that aren't 420x206px on the Ember Users page.

[ghost]

It would be awesome if all logos would be crisp hi-res images, because some look fuzzy (on a HiDPI screen) right now. Most importantly the top 12(?) logo's that also appear on the "Who's Using Ember.js?" slider on the front page.

I'm sure we can get a representive from these companies to submit a higher-res logo:

• [ ] Microsoft
• [ ] Heroku
• [ ] Skylight
• [ ] LinkedIn
• [ ] Square
• [ ] Blue Apron
• [ ] Kickstarter
• [ ] Codeschool

[MelSumner]

I believe this is a bug- we have good logos; the hdpi mixin might not be working properly due to some recent work on the site.