travis: force Python version

travis_custom/custom_build

@@ -6,6 +6,12 @@ set -x
 set -v
 
 # Setup environment variables.
+if [[ $TRAVIS_PYTHON_VERSION -gt 3 ]]; then
+  export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DPYTHON_EXECUTABLE=$(which python3)"
+else
+  export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DPYTHON_EXECUTABLE=$(which python2)"
+fi
+
 export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DBUILD_BENCHMARK=\"ON\" -DBUILD_TESTING=\"ON\" -DCMAKE_CXX_FLAGS=-DBOOST_SYSTEM_NO_DEPRECATED"
 if [[ ";${DO_INSTALL_DOC_EXCEPT_ON_BRANCH};" == *";${CI_BRANCH};"* ]]; then
   export CMAKE_ADDITIONAL_OPTIONS=" ${CMAKE_ADDITIONAL_OPTIONS} -DINSTALL_DOCUMENTATION=\"OFF\""