Support @import (inline) syntax in Less CSS 1.5

[davegurnell]

The new syntax is documented here.

The semantics of sbt-less don't need to change, but we should support the syntax.

[mdedetrich]

Good to see that this fixed #33 without us having to do anything!

Agree 0.7 is a good milestone for this, the new version of Less CSS 1.5 is still in beta