Measure commands execution time

Requires https://github.com/travis-ci/travis-build/pull/272, superseeds https://github.com/travis-ci/travis-build/pull/199 and https://github.com/travis-ci/travis-build/pull/263
cool, i didn't even know about `hash`

changed in https://github.com/travis-ci/travis-build/commit/d78eca34a77871da33f398ce8e839b9327fd84f5