Restart shell as login shell after rbenv

Move `exec $SHELL -l` line to spot recommended by [rbenv README][1].

Resolves [#77][2], where users would see errors like:

    Updating rubygems-update
    ERROR: While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /Library/Ruby/Gems/1.8
    directory.
    failed
    Installing critical Ruby gems for Rails development ...
    ERROR: While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-document
    failed
    Installing GitHub CLI client ...
    ERROR: While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-document
    failed

[1]: https://github.com/sstephenson/rbenv/
[2]: https://github.com/thoughtbot/laptop/issues/77