[BUILD_TRAVIS.py] additional line for nucleo_l053

Adding a line for nucleo_l053 target.
Merge branch 'build_travis_nucleo_l053' of github.com:ohagendorf/mbed into ohagendorf-build_travis_nucleo_l053

Conflicts:
	workspace_tools/build_travis.py
Merge branch 'ohagendorf-build_travis_nucleo_l053'