Bumped version to 3.0.7.3

Note this is the first time I needed to create a SHA256 hash of a file, I used (on a Mac):

```
shasum -a 256 MAMP_MAMP_PRO_3.0.7.3.pkg 
```

which gave me

```
840877041af7a06d50a3ddc67030ae4f193f70add473ebad0f7418daf80cd553
```

Also note that the file of the old version does not exist anymore, resulting in a HTTP 404:
http://downloads4.mamp.info/MAMP-PRO/releases/3.0.7.2/MAMP_MAMP_PRO_3.0.7.2.pkg