Fix issue with 'sed' command on Mac OS X.

When running the script on Mac OS X, the sed command expects a parameter with the -i flag that specifies the backup extension. In this case, no backup is needed, so it can be a blank string.

The following is the error:
```
Installing to /usr/local/bin/asciinema...
sed: 1: "/usr/local/bin/asciinema": extra characters at the end of l command
Oopsie, something unexpected happened.
```

The following article explains the issue in depth:
[sed in-place editing (-i) difference on Mac OS X](http://blog.mpdaugherty.com/2010/05/27/difference-with-sed-in-place-editing-on-mac-os-x-vs-linux/)

This fixes #36.