[FIX] Update pytest req'd version to 3.6

Fixes #2826.

Author: oesteban

nipype/info.py

@@ -108,7 +108,7 @@ def get_nipype_gitversion():
 SCIPY_MIN_VERSION = '0.14'
 TRAITS_MIN_VERSION = '4.6'
 DATEUTIL_MIN_VERSION = '2.2'
-PYTEST_MIN_VERSION = '3.0'
+PYTEST_MIN_VERSION = '3.6'
 FUTURE_MIN_VERSION = '0.16.0'
 SIMPLEJSON_MIN_VERSION = '3.8.0'
 PROV_VERSION = '1.5.2'