Better travis python testing.

.travis.yml

@@ -60,6 +60,8 @@ matrix:
             - boost-latest
           packages: 
             - libboost-python1.55-dev
+            - python3 
+            - python3-pip
             - *gcc5_pkgs
           
 cache:

dlib/travis/build-and-test.sh

@@ -16,16 +16,25 @@ if [ "$VARIANT" = "examples" ]; then
 fi
 
 if [ "$VARIANT" = "python-api" ]; then
-  pip install --user numpy
   ../cmake/bin/cmake ../tools/python -DCMAKE_BUILD_TYPE=Release
   ../cmake/bin/cmake --build . --target install -- -j 2
+  ../python_examples/svm_rank.py
 
+  rm -rf *
+  pip install --user numpy
+  ../cmake/bin/cmake ../tools/python -DCMAKE_BUILD_TYPE=Release
+  ../cmake/bin/cmake --build . --target install -- -j 2
   ../python_examples/svm_rank.py
 fi
 
 if [ "$VARIANT" = "python3-api" ]; then
   ../cmake/bin/cmake ../tools/python -DPYTHON3=ON -DCMAKE_BUILD_TYPE=Release
   ../cmake/bin/cmake --build . --target install -- -j 2
+  python3 ../python_examples/svm_rank.py
 
+  pip3 install --user numpy
+  rm -rf *
+  ../cmake/bin/cmake ../tools/python -DPYTHON3=ON -DCMAKE_BUILD_TYPE=Release
+  ../cmake/bin/cmake --build . --target install -- -j 2
   python3 ../python_examples/svm_rank.py
 fi