Be more specific in which python interpreter we want to run

.travis/install_macos.sh

@@ -5,5 +5,5 @@ wxmac opencv assimp libvorbis [email protected] || true
 brew install ode --with-shared
 
 # We can't trust brew to make the right symlinks, so execute commands as modules
-${PYTHON_INTERP:-python3} -m pip install virtualenv
-${PYTHON_INTERP:-python3} -m virtualenv --python=${PYTHON_INTERP:-python3} venv
+/usr/local/bin/${PYTHON_INTERP:-python3} -m pip install virtualenv
+/usr/local/bin/${PYTHON_INTERP:-python3} -m virtualenv --python=${PYTHON_INTERP:-python3} venv