Try installing ghostscript on travis

Tests are failing on travis, it might be because of this:

    /bin/sh: 2: gs: not found
    ...removing /home/travis/build/boostorg/build/boost/bin.v2/gs.check

So try installing ghostscript.

.travis.yml

@@ -30,6 +30,7 @@ addons:
       - dvipsk-ja
       - texlive
       - sshpass
+      - ghostscript
   ssh_known_hosts: frs.sourceforge.net
 
 env: