Create explicit 'pluggymaster' env definitions

For some reason, the previous approach brakes 'coveralls' because
pip still tries to install the 'pluggy' master requirement
(git+https://...)

tox.ini

@@ -21,14 +21,11 @@ envlist =
 [testenv]
 commands = pytest --lsof -ra {posargs:testing}
 passenv = USER USERNAME
-setenv=
-    pluggymaster: _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
 deps =
     hypothesis>=3.5.2
     nose
     mock
     requests
-    pluggymaster: git+https://github.com/pytest-dev/pluggy.git@master
 
 [testenv:py26]
 commands = pytest --lsof -ra {posargs:testing}
@@ -120,6 +117,24 @@ deps=numpy
 commands=
   pytest -ra {posargs:testing/python/approx.py}
 
+[testenv:py27-pluggymaster]
+passenv={[testenv]passenv}
+commands={[testenv]commands}
+setenv=
+    _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
+deps =
+    {[testenv]deps}
+    git+https://github.com/pytest-dev/pluggy.git@master
+
+[testenv:py35-pluggymaster]
+passenv={[testenv]passenv}
+commands={[testenv]commands}
+setenv=
+    _PYTEST_SETUP_SKIP_PLUGGY_DEP=1
+deps =
+    {[testenv]deps}
+    git+https://github.com/pytest-dev/pluggy.git@master
+
 [testenv:docs]
 skipsdist = True
 usedevelop = True