Allow 8.4pl6 to fail on travis

Apparently travis no longer supports Precise
(https://travis-ci.org/JasonGross/coq-tools/jobs/575163759), and I can't
get a working OCaml 3 package for Trusty, so I'm going to give up on Coq
8.4 compatibility testing.

.travis.yml

@@ -36,6 +36,7 @@ matrix:
       python: "2.7"
       env: COQ_VERSION="8.4pl6"   COQ_PACKAGE="coq-8.4pl6 libcoq-ocaml-8.4pl6" PPA="ppa:jgross-h/many-coq-versions-ocaml-3"
   allow_failures:
+    - env: COQ_VERSION="8.4pl6"   COQ_PACKAGE="coq-8.4pl6 libcoq-ocaml-8.4pl6" PPA="ppa:jgross-h/many-coq-versions-ocaml-3"
     - env: COQ_VERSION="v8.5"     COQ_PACKAGE="coq"                            PPA="ppa:jgross-h/coq-8.5-daily"
     - env: COQ_VERSION="v8.6"     COQ_PACKAGE="coq"                            PPA="ppa:jgross-h/coq-8.6-daily"
     - env: COQ_VERSION="v8.7"     COQ_PACKAGE="coq"                            PPA="ppa:jgross-h/coq-8.7-daily"