tests: add coverage tox factor, use it on Travis

.travis.yml

@@ -11,15 +11,14 @@ python:
 matrix:
   include:
     - python: 3.5
-      env: TOXENV=cov
-  allow_failures:
-    - env: TOXENV=cov
+      env: TOXENV=py35-coverage
 install:
     - pip install --quiet tox-travis
 script:
     - tox
 after_script:
-    - if [ $TOXENV == "cov" ]; then
-      pip install --quiet coveralls;
-      coveralls;
+    - |
+      if [ "${TOXENV%-coverage}" == "$TOXENV" ]; then
+        pip install --quiet coveralls;
+        coveralls;
       fi