Fix line numbers not matching up with code in Firefox.

Firefox has had broken line number alignment for a while now, if this is a bug with Firefox they don't seem to want to fix it since its been there from 46 through 49. This fixes it.

Before:

![Screen_Shot_2016-09-22_at_3.08.44_PM](/uploads/8bb65f40e44f738c0c375b0c8ef7dea6/Screen_Shot_2016-09-22_at_3.08.44_PM.png)

After:

![Screen_Shot_2016-09-22_at_3.09.10_PM](/uploads/9e537be56886569d8a82390f2b97c42b/Screen_Shot_2016-09-22_at_3.09.10_PM.png)

Fixes #20202.

Safari/Chrome work the same as before, Snippets seems to work fine still as well.

Page tested: http://localhost:3000/gitlab-org/gitlab-ce/blob/master/app/models/project.rb

cc: @annabeldunstone

See merge request !6485