travis: set default python version

.travis.yml

@@ -75,7 +75,7 @@ script:
   - mkdir build
   - cd build
   - export CMAKE_PREFIX_PATH=/tmp/_ci/install
-  - cmake ..
+  - cmake .. -DPYTHON_EXECUTABLE=$PYTHON_DEFAULT_VERSION
   - make
   - ./extra_lib
   - cd /tmp/_ci/build

travis_custom/custom_before_install

@@ -2,8 +2,10 @@
 
 # Add Python dependency
 echo "TRAVIS_PYTHON_VERSION=$TRAVIS_PYTHON_VERSION"
+export PYTHON_DEFAULT_VERSION=$(which python2)
 if [[ $TRAVIS_PYTHON_VERSION -gt 30 ]]; then
   export APT_DEPENDENCIES=$APT_DEPENDENCIES" python3-numpy"
+  export PYTHON_DEFAULT_VERSION=$(which python3)
 fi
 
 # When this script is called the current directory is ./custom_travis