Switch to regular breathe package on Travis

Breathe was previously installed from the git master because of a bug
in the released version. The fix has since migrated to PyPI.

[skip appveyor]

.travis.yml

@@ -68,11 +68,10 @@ matrix:
     env: DOCS STYLE LINT
     install:
     - export PATH="~/.local/bin:$PATH"
-    - $PY_CMD -m pip install --user --upgrade sphinx sphinx_rtd_theme flake8 pep8-naming
+    - $PY_CMD -m pip install --user --upgrade sphinx sphinx_rtd_theme breathe flake8 pep8-naming
     - |
       curl -fsSL ftp://ftp.stack.nl/pub/users/dimitri/doxygen-1.8.12.linux.bin.tar.gz | tar xz
       export PATH="$PWD/doxygen-1.8.12/bin:$PATH"
-      $PY_CMD -m pip install --user --upgrade https://github.com/michaeljones/breathe/archive/master.zip
     script:
     - make -C docs html SPHINX_OPTIONS=-W
     - tools/check-style.sh