Make travis build both python2 and python3 versions of the python extension.

.travis.yml

@@ -45,6 +45,22 @@ matrix:
           packages: 
             - libboost-python1.55-dev
             - *gcc5_pkgs
+    # build the python3 extension 
+    - compiler: gcc
+      env:
+        - GCC_VER=5
+        - ADDRESS_MODEL=64
+        - BUILD_SYSTEM=cmake
+        - PATH=$HOME/bin:$PATH
+        - VARIANT=python3-api
+      addons: &ao_gcc5
+        apt:
+          sources: 
+            - ubuntu-toolchain-r-test
+            - boost-latest
+          packages: 
+            - libboost-python1.55-dev
+            - *gcc5_pkgs
           
 cache:
   directories:

dlib/travis/build-and-test.sh

@@ -22,3 +22,11 @@ if [ "$VARIANT" = "python-api" ]; then
 
   ../python_examples/svm_rank.py
 fi
+
+if [ "$VARIANT" = "python3-api" ]; then
+  pip 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
+fi