try to fix travis again

add a new tox target "no-dilate" to use on py2.7, and use "coverage"
everywhere else

.travis.yml

@@ -17,10 +17,10 @@ before_script:
       flake8 *.py src --count --select=E901,E999,F821,F822,F823 --statistics ;
     fi
 script:
-  - if [[ $TRAVIS_PYTHON_VERSION == 3.7 ]]; then
-      tox -e coverage ;
+  - if [[ $TRAVIS_PYTHON_VERSION == 2.7 ]]; then
+      tox -e no-dilate ;
     else
-      tox -e py ;
+      tox -e coverage ;
     fi
 after_success:
   - codecov

tox.ini

@@ -18,7 +18,7 @@ commands =
     wormhole --version
     python -m wormhole.test.run_trial {posargs:wormhole}
 
-[testenv:py27]
+[testenv:no-dilate]
 extras = dev
 
 # on windows, trial is installed as venv/bin/trial.py, not .exe, but (at