Bump to v0.3.1.

Author: Hans Gaiser

setup.py

@@ -2,7 +2,7 @@
 
 setuptools.setup(
     name='keras-retinanet',
-    version='0.3',
+    version='0.3.1',
     description='Keras implementation of RetinaNet object detection.',
     url='https://github.com/fizyr/keras-retinanet',
     author='Hans Gaiser',