make carthage bootstrap step verbose

[rickpasetto]

In order to avoid Travis build failures (due to its 10 minute 'no output' timeout), this ensures that the carthage bootstrap step outputs something while it is working.

[darinkrauss]

It looks like this also includes some changes to enable caching, yes? If so, please include this in the final merge commit notes (as it is probably more important that the verbose logs).

Hopefully we can remove these verbose logs once we get the build straightened out.

[rickpasetto]

My apologies, @darinkrauss . You are right -- these two things should have been separate PRs. The --verbose PR was solely an attempt to fix Travis builds (which were timing out). The "bootstrap-if-needed" strategy was not related; it was part of my work to cache builds (specifically for CircleCI). I accidentally put the latter change into this branch. Will fix it.