Fix public URL generation in LiipImagineThumbnail

LiipImagineThumbnail::generatePublicUrl returns a path with two leading slashes (one added by Router::generate and the other by ImageProvider::getCdnPath), which at the end results in a path with three leading slashes, because ImageProvider::getCdnPath gets called again by ImageProvider::generatePublicUrl. 
FormatThumbnail::generatePublicUrl is an example of a working thumbnail provider that returns a path with no leading slashes.

The issue is documented here: https://groups.google.com/d/msg/sonata-users/xstEkej6TqY/ZcNYvGLxNQEJ