Fix: Do not update composer itself more often than once

[localheinz]

This PR

• [x] removes a redundant run of composer self-update from .travis.yml

💁‍♂️ Travis updates composer twice

• https://travis-ci.org/beberlei/assert/jobs/208301839#L135
• https://travis-ci.org/beberlei/assert/jobs/208301839#L141

and we update it once more

• https://travis-ci.org/beberlei/assert/jobs/208301839#L173

[rquadling]

Thank you.

[localheinz]

Thank you, @rquadling!