Merge extra and main test envs, update tox command

- coverage was not picking up by coveralls, because I removed the
  coverage package from deps while removing py27 config
- remove PYTHONHASHSEED variable, because it is not necessary in Python
  3
- remove extra env from travis config

.travis.yml

@@ -21,8 +21,6 @@ jobs:
   include:
     - python: 3.8
       env: TOX_ENV=py38
-    - python: 3.8
-      env: TOX_ENV=extra
     - python: 3.7
       env: TOX_ENV=py37
     - python: 3.6

tox.ini

@@ -1,29 +1,26 @@
 [tox]
-envlist = py35,py36,py37,py38,extra,flake8,docs
-
-[testenv:extra]
-deps = 
-    {[testenv]deps}
-    circus-web
-    gevent
-    papa
+envlist = py35,py36,py37,py38,flake8,docs
 
 [testenv]
 passenv = PWD
 deps =
     nose
+    nose-cov
+    coverage
     mock
+    circus-web
+    gevent
+    papa
     PyYAML
     tornado>=3.0,<5.0
     pyzmq>=17.0
 
 setenv =
     TESTING=1
-    PYTHONHASHSEED=random
     PYTHONUNBUFFERED=1
 
 commands =
-    nosetests -vs circus/tests
+    nosetests -vs --with-coverage --cover-package=circus circus/tests
 
 
 [testenv:docs]