pass MB_PYTHON_VERSION to docker run in build_multilinux

It is useful sometimes (e.g. in config.sh's pre_build') to do stuff based on
the current python version. The problem is that when running inside the docker
container the variable MB_PYTHON_VERSION is passed on only as PYTHON_VERSION
without the MB_ prefix, but in the macOS environment the latter is not
present. Thus, one needs to check both PYTHON_VERSION and MB_PYTHON_VERSION.
Better to pass both to the docker container, so one can jut check the value
of MB_PYTHON_VERSION on both mac and linux.

Note that the same hack is used in the 'install_run' function, where both
PYTHON_VERSION and MB_PYTHON_VERSION are passed through to docker run.

travis_linux_steps.sh

@@ -84,6 +84,7 @@ function build_multilinux {
     docker run --rm \
         -e BUILD_COMMANDS="$build_cmds" \
         -e PYTHON_VERSION="$MB_PYTHON_VERSION" \
+        -e MB_PYTHON_VERSION="$MB_PYTHON_VERSION" \
         -e UNICODE_WIDTH="$UNICODE_WIDTH" \
         -e BUILD_COMMIT="$BUILD_COMMIT" \
         -e CONFIG_PATH="$CONFIG_PATH" \