add 'tox -e docs', travis job to exercise it

.travis.yml

@@ -31,5 +31,9 @@ matrix:
     - python: 3.6
       script: tox -e flake8less
       env: FLAKE8
+    - python: 3.7
+      dist: xenial
+      script: tox -e docs
+      env: DOCS
   allow_failures:
     - python: nightly

tox.ini

@@ -44,3 +44,11 @@ max-complexity = 40
 [testenv:flake8less]
 deps = flake8
 commands = flake8 --select=E901,E999,F821,F822,F823 src/wormhole
+
+[testenv:docs]
+deps =
+     sphinx
+     recommonmark
+skip_install = True
+commands =
+         sphinx-build -b html -d {toxinidir}/docs/_build/doctrees {toxinidir}/docs {toxinidir}/docs/_build/html