If `git remote add` fails, fallback to `git remote set-url`.

Avoids git error `fatal: remote origin already exists.`

```shell
$ ruby ./install
==> This script will install:
/tmp/homebrew-test/bin/brew
/tmp/homebrew-test/Library/...
/tmp/homebrew-test/share/man/man1/brew.1

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/mkdir /tmp/homebrew-test
Password:
==> /usr/bin/sudo /bin/chmod g+rwx /tmp/homebrew-test
==> /usr/bin/sudo /usr/bin/chgrp admin /tmp/homebrew-test
==> /usr/bin/sudo /bin/chmod g+rwx /Library/Caches/Homebrew
==> Downloading and installing Homebrew...
fatal: remote origin already exists.
```
Changing to 
```ruby
(system git, "remote", "add", "origin", "https://github.com/Homebrew/homebrew") || (system git, "remote", "set-url", "--add", "origin", "git://github.com/Homebrew/homebrew.git")
```
gets me
```shell
==> This script will install:
/tmp/homebrew-test/bin/brew
/tmp/homebrew-test/Library/...
/tmp/homebrew-test/share/man/man1/brew.1

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/mkdir /tmp/homebrew-test
Password:
==> /usr/bin/sudo /bin/chmod g+rwx /tmp/homebrew-test
==> /usr/bin/sudo /usr/bin/chgrp admin /tmp/homebrew-test
==> /usr/bin/sudo /bin/chmod g+rwx /Library/Caches/Homebrew
==> Downloading and installing Homebrew...
fatal: remote origin already exists.
Failed during: git remote add origin https://github.com/Homebrew/homebrew
```

`system` is calling <https://github.com/jdhealy/homebrew/blob/29612e58b619feda9629d7b648bcf08797bb5839/install#L34>, not <http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-system>, right?