Try to install a python-version-appropriate Sphinx version.

Also upbate the .travis.yml to pick up modern versions of PyPy and PyPy3

.travis.yml

 language: python
-sudo: false
-matrix:
-    include:
-        - os: linux
-          python: pypy
-        - os: linux
-          python: pypy3
-          env: BUILOUT_OPTIONS=sphinx:eggs=
-        - os: linux
-          python: 2.7
-        - os: linux
-          python: 3.4
-        - os: linux
-          python: 3.5
-        - os: linux
-          python: 3.6
-        - python: 3.7
-          dist: xenial
-          sudo: true
+dist: xenial
+python:
+  - 2.7
+  - 3.4
+  - 3.5
+  - 3.6
+  - 3.7
+  - pypy
+  - pypy3
+
 install:
     - pip install -U pip
     - pip install -U setuptools zc.buildout

buildout.cfg

@@ -49,10 +49,16 @@ eggs =
     ${coverage-test:eggs}
 interpreter = py
 
+[sphinx_egg]
+eggs = Sphinx
+
+[sphinx_egg:python2 or python34]
+eggs = Sphinx < 2
+
 [sphinx]
 recipe = zc.recipe.egg
 eggs =
-    Sphinx
+    ${sphinx_egg:eggs}
     docutils
     ZODB
     j1m.sphinxautointerface