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
In the interest of not spawning sub-processes and instead use shell builtins when we can, I think this should be this instead;

``` bash
if hash gdate 2>/dev/null
```