Do not fail tox run if coveralls push fails

When tox environments are run locally, coveralls exits with an error
code as it is not authenticated to push to coveralls. Adding the - in
front allows tox to ignore a fail on this command and not mark the test
case as failed.

tox.ini

@@ -18,7 +18,7 @@ deps =
 commands =
     {envpython} --version
     nosetests -sv --with-coverage --cover-package=pelican pelican
-    coveralls
+    - coveralls
 
 [testenv:docs]
 basepython = python2.7