Fix the pip installation error.

https://travis-ci.org/github/google/qkeras/builds/753458315
```
Installed /home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/QKeras-0.8.0-py3.7.egg
Processing dependencies for QKeras==0.8.0
error: numpy 1.16.4 is installed but numpy>=1.16.5 is required by {'scipy'}
The command "python setup.py install" failed and exited with 1 during .
Your build has been stopped.
```
PiperOrigin-RevId: 350828042
Change-Id: I6214ec31edf208a0d55019495afb7cb2dd1fbacb

requirements.txt

@@ -1,5 +1,5 @@
 tensorflow>=2.1.0rc0, <2.4.0rc0
-numpy>=1.16
+numpy>=1.16.5
 pyparser
 scipy>=1.4.1
 setuptools>=41.0.0