CVMFS Client fails on mac OS Sierra 10.12

CVMFS determines the Mac OS client version, by examining /System/Library/CoreServices/SystemVersion.plist.

```
inline void platform_get_os_version(int32_t *major,
int32_t *minor,
int32_t *patch) {
const std::string plist = "/System/Library/CoreServices/SystemVersion.plist";
const std::string plist_key = "ProductVersion";

...
const int matches = sscanf(version.c_str(), "%u.%u.%u", major, minor, patch);
assert(matches == 3 && "failed to read OS X version string");
}
```

This is a hack, as major OS X releases consist of only a major/minor release, not a patch level.

```
<key>ProductVersion</key>
<string>10.12</string>
```

I'd highly appreciate if you could relax the requirement, to accept two dots only, and prepare a hot fix release.