Use github downloads for specific versions on Py2.7

.travis.yml

@@ -23,10 +23,11 @@ before_install:
     - travis_retry pip install wheel flake8 coveralls nose
 
     # on Python 2.7, use the system versions of numpy, scipy, and matplotlib
-    # and the minimum version of the rest of the requirements
+    # and the minimum version of cython and networkx
     -  if [[  $TRAVIS_PYTHON_VERSION == 2.7* ]]; then
             travis_retry sudo apt-get install python-scipy python-matplotlib;
-            pip install cython==0.19.2 networkx==1.6.2 six==1.3;
+            pip install https://github.com/cython/cython/archive/0.19.2.tar.gz
+            pip install https://github.com/networkx/networkx/archive/networkx-1.6.tar.gz
        fi
 
     - travis_retry pip install  -r requirements.txt $WHEELHOUSE;

requirements.txt

@@ -3,5 +3,5 @@ matplotlib>=1.1.1
 numpy>=1.6.1
 scipy>=0.9
 six>=1.3
-networkx>=1.6.2
+networkx>=1.6
 pillow>=1.1.7