Specify tensorflow version less than 2.0.0

Closes #150

mhcflurry/version.py

-__version__ = "1.4.0"
+__version__ = "1.4.1"

requirements.txt

@@ -2,7 +2,7 @@ six
 numpy>=1.11
 pandas>=0.20.3
 Keras>=2.2.5
-tensorflow>=1.1.0
+tensorflow>=1.1.0,<2.0.0
 appdirs
 scikit-learn
 mhcnames

setup.py

@@ -53,7 +53,7 @@ if __name__ == '__main__':
         'pandas>=0.20.3',
         'Keras>=2.2.5',
         'appdirs',
-        'tensorflow>=1.1.0',
+        'tensorflow>=1.1.0,<2.0.0',
         'scikit-learn',
         'mhcnames',
         'pyyaml',