Tell the python3 tests to use python3

dlib/travis/build-and-test.sh

@@ -24,9 +24,9 @@ if [ "$VARIANT" = "python-api" ]; then
 fi
 
 if [ "$VARIANT" = "python3-api" ]; then
-  pip install --user numpy
+  pip3 install --user numpy
   ../cmake/bin/cmake ../tools/python -DPYTHON3=ON -DCMAKE_BUILD_TYPE=Release
   ../cmake/bin/cmake --build . --target install -- -j 2
 
-  ../python_examples/svm_rank.py
+  python3 ../python_examples/svm_rank.py
 fi