Pin keras version

Pin keras version. Also add note on KERAS_BACKEND to README.md.

Closes #67. Closes #65.

README.md

@@ -13,7 +13,16 @@ The MHCflurry predictors are implemented in Python using [keras](https://keras.i
 
 ## Setup
 
-Install the package:
+To configure keras, the neural network library used by MHCflurry, you'll need to set an environment variable in your shell:
+
+```
+export KERAS_BACKEND=theano
+```
+
+If you're familiar with keras, you may also try using the tensorflow backend. MHCflurry is currently tested using theano, however.
+ 
+
+Now install the package:
 
 ```
 pip install mhcflurry

requirements.txt

@@ -2,7 +2,7 @@ numpy>= 1.11
 pandas>=0.13.1
 appdirs
 theano>=0.8.2
-keras
+keras==1.1.0
 fancyimpute>=0.0.12
 scikit-learn
 h5py

setup.py

@@ -76,7 +76,7 @@ if __name__ == '__main__':
             'pandas>=0.13.1',
             'appdirs',
             'theano>=0.8.2',
-            'keras',
+            'keras==1.1.0',
             'fancyimpute>=0.0.12',
             'scikit-learn',
             'h5py',