Fix install script for OS X

The install script [fails](https://github.com/lukapusic/soundcloud-dl/issues/61) on OS X because `/opt/local/bin` does [not exist](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man7/hier.7.html) on OS X. Using an existing directory resolves the issue.

Fortunately, `/usr/bin/` [exists](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man7/hier.7.html) in OS X, allowing us to stick to the pattern used for other *nix distributions.