Update requirements.txt
Update global requirements.txt for tests to pass.
+# Requirements for all census samples.
tensorflow==1.8
+# Requirements for keras example only for sample.sh.
+future==0.16.0
+h5py>=2.7.0,<3.0
+Keras>=2.2.4,<3.0
+numexpr>=2.6.8,<3.0
+pandas>=0.23.4,<1.0
