Travis CI: Add pytest --doctest-modules neural_network

Fixes TheAlgorithms#987
```
neural_network/perceptron.py:123: in <module>
    sample.insert(i, float(input('value: ')))
../lib/python3.7/site-packages/_pytest/capture.py:693: in read
    raise IOError("reading from stdin while output is captured")
E   OSError: reading from stdin while output is captured
-------------------------------------------------------------------------------- Captured stdout --------------------------------------------------------------------------------
('\nEpoch:\n', 399)
------------------------

value:
```

Author: cclauss

.travis.yml

@@ -25,6 +25,7 @@ script:
       linear_algebra_python
       matrix
       networking_flow
+      neural_network
       other
       searches
       sorts