Don't upgrade brew that is already to up to date

On a clean OS X Mavericks, Laptop outputs an error message towards the end of
the install script when `brew_install_or_upgrade` is invoked for OpenSSL:

    Upgrading and linking OpenSSL ...
    Error: openssl-1.0.1h already installed
    Linking /usr/local/Cellar/openssl/1.0.1h... 1139 symlinks created

The current version of OpenSSL was installed earlier as a dependency of
PostgreSQL. In general, a similar error will be logged whenever
`brew_install_or_upgrade` is called for an installed package that's already up
to date.

Currently, Laptop discards the error exit code of `brew upgrade` as follows:

    (brew upgrade "$@") || true

In addition to logging an error when there isn't truly an error, this approach
can cause real `brew upgrade` errors and potential bugs in Laptop to be masked
(e.g. Homebrew formula build failures or Laptop mistakenly invoking `brew
upgrade` for a package that isn't actually installed).

I think a better approach would be to check whether the installed package is the
same version as the current Homebrew formula before attempting `brew upgrade`.