Add composer self-update to travis config

Just trying to find an error with dependencies.

If someone is more interested: https://github.com/composer/composer/compare/7755564962718189d5d7d9fdee595283c8f032b7...master
There seems to be a BC in composer between that commit (7755564962718189d5d7d9fdee595283c8f032b7) and now.