tox: Set passenv for tox 2.0.

tox.ini

@@ -17,6 +17,7 @@ usedevelop = true
 [testenv:unittests]
 # https://bitbucket.org/hpk42/tox/issue/246/
 setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envdir}/lib/python3.4/PyQt5/plugins/platforms
+passenv = DISPLAY XAUTHORITY HOME
 deps =
     py==1.4.27
     pytest==2.7.0
@@ -31,6 +32,7 @@ commands =
     {envpython} -m py.test --strict {posargs}
 
 [testenv:coverage]
+passenv = DISPLAY XAUTHORITY HOME
 deps =
     {[testenv:unittests]deps}
     coverage==3.7.1
@@ -64,6 +66,7 @@ commands =
 [testenv:pep257]
 skip_install = true
 deps = pep257==0.5.0
+passenv = LANG
 # Disabled checks:
 # D102: Docstring missing, will be handled by others
 # D209: Blank line before closing """ (removed from PEP257)