Banner updates for Pro free trial messaging

I like free software. I also like having the occasional paycheck so my kids don't complain about hunger pangs.

## Verification

### Rotating banner blurbs

 - [x] Run `msfconsole`
 - [x] **On a git source checkout**: You should **not** see the rotating blurbs

 - [x] Copy `core.rb` to an installed binary version of Metasploit Community Edition on Windows, in `C:/metasploit/app/pro/msf3/lib/msf/ui/console/command_dispatcher/`
 - [x] **On a Windows install**: You should see the rotating blurbs when you start `msfconsole`

 - [x] Copy `core.rb` to an installed binary version of Metasploit Community Edition on Linux, in `/opt/metasploit/app/pro/msf3/lib/msf/ui/console/command_dispatcher/`
 - [x] **On a Linux install**: You should see the rotating blurbs when you start `msfconsole`

### Static link to Metasploit Pro download

 - [x] Run `msfconsole`
 - [x] You should see the banner trailer with the shortlink:

````
       =[ metasploit v4.9.2-dev [core:4.9 api:1.0]        ]
+ -- --=[ 1294 exploits - 708 auxiliary - 204 post        ]
+ -- --=[ 335 payloads - 35 encoders - 8 nops             ]
+ -- --=[ Free Metasploit Pro trial: http://r-7.co/trymsp ]
````

 - [x] Visiting the shortlink should lead to a Metasploit download page.