new sphinx (#1786)

diff --git a/.travis.yml b/.travis.yml
index e1209b6..4cc5cf0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,8 +15,8 @@
     - pyenv global $(pyenv whence 2to3)  # activate all python versions
     - PY_CMD=python3
     - $PY_CMD -m pip install --user --upgrade pip wheel setuptools
-    install: # Breathe does not yet support Sphinx 2
-    - $PY_CMD -m pip install --user --upgrade "sphinx<2" sphinx_rtd_theme breathe flake8 pep8-naming pytest
+    install:
+    - $PY_CMD -m pip install --user --upgrade sphinx sphinx_rtd_theme breathe flake8 pep8-naming pytest
     - curl -fsSL https://sourceforge.net/projects/doxygen/files/rel-1.8.15/doxygen-1.8.15.linux.bin.tar.gz/download | tar xz
     - export PATH="$PWD/doxygen-1.8.15/bin:$PATH"
     script: