The sw_vers -productVersion | sed "s/$10.[0-9]$.*/\1/" line in osx.yaml will stop working for OS X 10.10

The line at https://github.com/hashdist/hashstack/blob/master/osx.yaml#L8:

```
    export MACOSX_DEPLOYMENT_TARGET=$(sw_vers -productVersion | sed "s/$10.[0-9]$.*/\1/")
```

Will stop working once the OS X version is 10.10. So it needs to be improved.

Reported by @asmeurer.
