added requirements for Python 2.6, removed nose

borrowed from geopandas

Author: micahcochran

.travis.yml

@@ -33,7 +33,9 @@ install:
     pip install $NUMPY
     pip install $MPL
     pip install -r requirements.txt
-    pip install nose
+    
+  - if [[ $TRAVIS_PYTHON_VERSION == '2.6' ]]; then pip install -r .requirements-2.6.txt; fi
+  
   - |
     cd geos-3.3.3
     export GEOS_DIR=$HOME/.local/