Update Synergy to 1.5.1

SHA256 calculated with:

```bash
$ openssl dgst -sha256 synergy-1.5.1-r2398-MacOSX109-x86_64.dmg
SHA256(synergy-1.5.1-r2398-MacOSX109-x86_64.dmg)= d787031705b0123d36f07e7639fd8882d1969bc8f7ddad25a3326c873385dfe2
```