More fixes for travis tests

acme/acme/challenges.py

@@ -245,7 +245,7 @@ class DNS01Response(KeyAuthorizationChallengeResponse):
             from acme import dns_resolver
             txt_records = dns_resolver.txt_records_for_name(
                 validation_domain_name)
-        except ImportError:
+        except ImportError:  # pragma: no cover
             raise ImportError("Local validation for 'dns-01' challenges "
                               "requires 'dnspython'")
         exists = validation in txt_records

tox.ini

@@ -15,7 +15,7 @@ envlist = py26,py27,py33,py34,py35,cover,lint
 # packages installed separately to ensure that dowstream deps problems
 # are detected, c.f. #1002
 commands =
-    pip install -e acme[testing]
+    pip install -e acme[dns,testing]
     nosetests -v acme
     pip install -e .[testing]
     nosetests -v letsencrypt