get_iplayer Windows installer updated to 4.8

[dinkypumpkin]

The get_iplayer Windows installer has been updated in response to the 
Windows 8.1 issue raised in this thread:

http://www.mail-archive.com/get_iplayer@lists.infradead.org/msg05052.html

Windows 8.1 users should re-install get_iplayer with the updated 
installer. Read the information here:

https://github.com/dinkypumpkin/get_iplayer/wiki/win81

If you do not have Windows 8.1, ignore this.  You *do not* need to 
re-install get_iplayer unless you use Windows 8.1.