Fix basepython for tox envs "doctesting" and "coveralls"

Because we are currently using "nighly" python for travis, which
uses python 3.6 as the default interpreter. This breaks the environments
listed above because "inspect.getargspec" has been removed in py36.

tox.ini

@@ -89,6 +89,7 @@ commands=
     make html
 
 [testenv:doctesting]
+basepython = python3.4
 changedir=doc/en
 deps=PyYAML
 commands= py.test -rfsxX {posargs}
@@ -117,6 +118,7 @@ commands=
     
 
 [testenv:coveralls]
+basepython = python3.4
 changedir=testing
 deps =
     {[testenv]deps}