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
This is going to shell out every single time `travis_nanoseconds` is called, which will probably add up to some time over the course of a build. I just realised that OS X `date` will output the `%N` verbatim, so we could do something like this:

``` bash
function travis_nanoseconds() {
    if hash gdate 2>/dev/null; then
      gdate -u "+%s%N"
    else
      out="$(date -u "+%s%N")"
      echo "${out/%N/000000000}"
    fi
}
```

This seems to work fine on my machine without `gdate` and BSD-style `date`.