[MXNET-908] Enable python tests in Travis (#12550)

This turns on python tests in Travis in order to increase or MacOS coverage.

.travis.yml

@@ -29,3 +29,4 @@ script:
   - export MXNET_STORAGE_FALLBACK_LOG_VERBOSE=0
   - mv make/osx.mk config.mk
   - make -j 2
+  - python -m nose --verbose tests/python/unittest/

ci/travis/install.sh

@@ -25,6 +25,6 @@ if [ ${TRAVIS_OS_NAME} == "osx" ]; then
     brew install libpng
     brew install ImageMagick
     brew install swig
-	python -m pip install --user nose numpy cython scipy requests
-	python3 -m pip install --user nose numpy cython scipy requests
+    python -m pip install --user nose numpy cython scipy requests mock
+    python3 -m pip install --user nose numpy cython scipy requests mock
 fi