Fix SSL certificate verification issue

Detected while using rosdep (ros-infrastructure/rosdep#808)

.travis.yml

@@ -15,7 +15,7 @@ install:
   - python setup.py build develop
   # newer versions of docutils dropped support for Python 3.4
   - if [ $TRAVIS_PYTHON_VERSION == "3.4" ]; then pip install docutils==0.15.2; fi
-  - pip install PyYAML argparse catkin_pkg rospkg setuptools
+  - pip install PyYAML argparse certifi catkin_pkg rospkg setuptools
   - pip install nose coverage mock
 # command to run tests
 script:

src/rosdistro/loader.py

@@ -42,10 +42,14 @@
     from urllib2 import HTTPError
     from urllib2 import URLError
 
+# to fix errors with SSL certificate verification
+import ssl
+import certifi
 
 def load_url(url, retry=2, retry_period=1, timeout=10, skip_decode=False):
     try:
-        fh = urlopen(url, timeout=timeout)
+        fh = urlopen(url, timeout=timeout,
+                     context=ssl.create_default_context(cafile=certifi.where()))
     except HTTPError as e:
         if e.code in [500, 502, 503] and retry:
             time.sleep(retry_period)