Remove --python flag since we know we're running the proper python

.travis/install_macos.sh

@@ -6,4 +6,4 @@ brew install ode --with-shared
 
 # We can't trust brew to make the right symlinks, so execute commands as modules
 /usr/local/bin/${PYTHON_INTERP:-python3} -m pip install virtualenv
-/usr/local/bin/${PYTHON_INTERP:-python3} -m virtualenv --python=${PYTHON_INTERP:-python3} venv
+/usr/local/bin/${PYTHON_INTERP:-python3} -m virtualenv venv