Test with mtacceptor too on Python 3.

(There seems to be a resource issue with Python 2, that I might run
down some time.)

.travis.yml

@@ -8,6 +8,12 @@ matrix:
           python: 3.4
         - os: linux
           python: 3.5
+        - os: linux
+          python: 3.4
+          env: ZEO_MTACCEPTOR=1
+        - os: linux
+          python: 3.5
+          env: ZEO_MTACCEPTOR=1
 install:
     - pip install -U setuptools
     - python bootstrap.py