[tern addon] Update CSS to prevent cutting off of icons

I noticed that the image that accompanies auto-completion suggestions
when the tern addon is active gets cut slightly, because the default
line height is slightly smaller than the image height.

This change above ensures that the line height is bigger, hence
ensuring that the image is fully shown.

addon/tern/tern.css

 .CodeMirror-Tern-completion {
   padding-left: 22px;
   position: relative;
+  line-height: 1.5;
 }
 .CodeMirror-Tern-completion:before {
   position: absolute;