Use Homebrew package name (not alias) for Postgres

Fixes https://github.com/thoughtbot/laptop/issues/258

The following previously did not behave as intended:

```
brew_install_or_upgrade 'postgres'
```

because "postgres" is an alias, not the full name of the formula. The actual
formula name is "postgresql".

The first time `brew_install_or_upgrade 'postgres'` is invoked it works as
expected — `brew install postgres` is run. Additional invocations result in
`brew install` instead of the expected `brew upgrade`.

The `brew_install_or_upgrade` function uses `brew list -1` to obtain a complete
list of installed packages, and these package names will be the actual package
names. So, grepping for an alias (with the `-x` option) won't match:

``` Bash
$ brew list -1 | grep -Fx postgres
```

Note that there's no output, even though PostgreSQL is installed. Using the full
package name behaves as expected:

``` Bash
$ brew list -1 | grep -Fx postgresql
postgresql
```

Rather than passing on the first argument to `brew_install_or_upgrade` to the
`brew list` commands, the argument should first be expanded to the actual
package name. (I'm working on a PR to this effect.)