⬇️ Use tighter dependencies for TensorFlow and Calamari

With the recent release of TensorFlow 2.0, we need to make sure we
install a version compatible with the current Calamari release, e.g.
TF < 2.0.

Also, depend on a Calamari version that is known to be working, that
is Calamari >= 0.3.5.

Fixes #6.

requirements.txt

@@ -1,6 +1,6 @@
 numpy
-calamari-ocr
+tensorflow-gpu < 2.0
+calamari-ocr >= 0.3.5
 setuptools >= 41.0.0  # tensorboard depends on this, but why do we get an error at runtime?
-tensorflow-gpu
 click
 ocrd >= 1.0.0b11