Moved the caveats message after the finish step

I changed a little bit the order of appearance of the messages at the end of an installation.

Previously you had :

fi.caveats
fi.finish (and at the end of finish, call to summary)

Now :

fi.finish
fi.caveats
fi.summary

There are two reasons for my change :

1) The finish step can be a quite long to process, for example when linking libraries like VTK or ITK. In this case the caveats message was displayed, nothing happened then, and finally the summary was displayed. I was fooled by this once or two, thinking the installation was stuck somewhere after the caveats.
2) When working in verbose mode, you would previously get a lot of linking messages (ln -s) (+ other messages) between the caveats and the summary. I think it is better to have the caveats at the end, just before the summary.

I hope I did not break anything here, it's my first PR for a more critical part of homebrew other than a formula :)