[css] Failed to decode downloaded font, OTS parsing error: invalid version tag + rails 4

try

@font-face {
  font-family: 'icomoon';
  src: asset-url('icomoon.eot');
  src: asset-url('icomoon.eot?#iefix') format('embedded-opentype'),
       asset-url('icomoon.ttf') format('truetype'),
       asset-url('icomoon.woff') format('woff'),
       asset-url('icomoon.svg#icomoon') format('svg');
  font-weight: normal;
  font-style: normal;
}

and rename your file to application.css.scss