travis: Update ruby to 2.4 for latest github-pages
travis: Update ruby to 2.4 for latest github-pages

Signed-off-by: BJ Hargrave <3d26bb3930946e3370762d769a073cece444c2d8@bjhargrave.com>

psoreide/bnd,psoreide/bnd,mcculls/bnd,mcculls/bnd,mcculls/bnd,psoreide/bnd