Upgrade Lanterna from 2.1.1 to 2.1.5

During the course of using clojure-lanterna I ran into a nasty bug in
Lanterna where `clear`ing my screen left behind white background
artifacts which persisted across multiple subsequent `redraw`s. Upgrading
Lanterna to 2.1.5 corrected this issue for me.

This pull-request upgrades the Lanterna dependency to 2.1.5 and bumps
the project's version to 0.9.4.