ncurses dependency update from 5 to 6

Related to libmenuw.so.5: cannot open shared object file: No such file or directory #485

As of 2015-08-08 ncurses switched to major version 6, rendering any already-build ncursesw gem unusable (as well as sup). The change hit Archlinux on 2015-09-06, other distributions will surely be affected not long after.

The solution is to reinstall it from scratch so it gets rebuilt with the new headers:

$ gem uninstall ncursesw
$ gem install ncursesw