Fixed compile problem with LESS 1.2.1.

Fixed compile problem with LESS 1.2.1.

The sprite path variables introduced in v2.0.1 prevents bootstrap.less to
compile wiht LESS v1.2.1. Updated the variables to use the @{name}
construct when inserted in sprites.less, as outlined in the LESS
documentation. This change makes the CSS compile with LESS v1.2.1.