travis: Disable testing on gentoo

We would love to test on gentoo, but this currently breaks CI. Open to
community contributions to improve our coverage of this distrubition.

.travis.yml

@@ -2,10 +2,6 @@ sudo: required
 language: generic
 matrix:
   include:
-    - name: "Gentoo (latest)"
-      os: linux
-      env: CONTAINER=gentoo
-      services: docker
     - name: "Centos 7"
       os: linux
       env: CONTAINER=centos7