[travis/tox] check syntax of reStructuredText before release

I pushed a NEWS.rst with broken indentation, the PyPI page will
display plan text garbage.

Next time I'll use docutils and do `python setup.py check
--restructuredtext` before pushing a new tag.

Also, I forgot to update .travis.yml to build the dist packages
on python3.6 instead of 3.5 after I modified the tox.ini, so
the automatic release from Travis failed.

This time I had to use twine tool to upload the zip and whl
packages from my local machine to PyPI.

Sorry the mess...

.travis.yml

@@ -10,11 +10,11 @@ matrix:
     - python: 3.4
       env: TOXENV=py34-cov
     - python: 3.5
-      env:
+      env: TOXENV=py35-cov
-        - TOXENV=py35-cov
-        - BUILD_DIST=true
     - python: 3.6
-      env: TOXENV=py36-cov
+      env:
+        - TOXENV=py36-cov
+        - BUILD_DIST=true
     - python: pypy2.7-5.8.0
       # disable coverage.py on pypy because of performance problems
       env: TOXENV=pypy-nocov

tox.ini

@@ -46,6 +46,8 @@ commands =
 [testenv:bdist]
 basepython = {env:TOXPYTHON:python3.6}
 deps =
+    pygments
+    docutils
     setuptools
     wheel
 skip_install = true
@@ -55,6 +57,8 @@ install_command =
 whitelist_externals =
     rm
 commands =
+    # check metadata and rst long_description
+    python setup.py check --restructuredtext --strict
     # clean up build/ and dist/ folders
     rm -rf {toxinidir}/dist
     python setup.py clean --all