update homebrew instructions

content/install.txt

@@ -228,17 +228,17 @@ you are safe.
 
 4. Install a few prerequisites ::
 
-       brew install gfortran python
+       brew install gcc python
 
 5. Add additional repositories ::
 
        brew tap homebrew/science
-       brew tap samueljohn/python
+       brew tap homebrew/python
        brew tap michaelwimmer/kwant
 
 6. Install Kwant and its prerequisites ::
 
-       pip install nose
+       pip install nose six
        brew install numpy scipy matplotlib
        brew install kwant