add aggressive version detection for SilverStripe

Hi there,

I've added aggressive version detection for SilverStripe. I will be testing these more in the future, but for now it sort of works. Below are the results of tests:

<pre>
for version in `ls -la | grep drw | awk '{print $9}' | grep -vE '^\.'`; do ~/WhatWeb/whatweb -a 4 --follow-redirect=never -p ~/WhatWeb/plugins/silverstripe.rb http://localhost/$version/; done
http://localhost/2.4.10/ [302] SilverStripe[2.4.10,2.4.11,2.4.12,2.4.13,2.4.9]
http://localhost/2.4.11/ [302] SilverStripe[2.4.10,2.4.11,2.4.12,2.4.13,2.4.9]
http://localhost/2.4.12/ [302] SilverStripe[2.4.10,2.4.11,2.4.12,2.4.13,2.4.9]
http://localhost/2.4.13/ [302] SilverStripe[2.4.10,2.4.11,2.4.12,2.4.13,2.4.9]
http://localhost/2.4.9/ [302] SilverStripe[2.4.10,2.4.11,2.4.12,2.4.13,2.4.9]
http://localhost/3.0.0/ [302] SilverStripe[3.0.0]
http://localhost/3.0.1/ [302] SilverStripe[3.0.1]
http://localhost/3.0.10/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.0.11/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.0.3/ [302] SilverStripe[3.0.3]
http://localhost/3.0.4/ [302] SilverStripe[3.0.4,3.0.5]
http://localhost/3.0.5/ [302] SilverStripe[3.0.4,3.0.5]
http://localhost/3.0.6/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.0.7/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.0.8/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.0.9/ [302] SilverStripe[3.0.10,3.0.11,3.0.6,3.0.7,3.0.8,3.0.9]
http://localhost/3.1.0/ [302] SilverStripe[3.1.0,3.1.1]
http://localhost/3.1.1/ [302] SilverStripe[3.1.0,3.1.1]
http://localhost/3.1.2/ [302] SilverStripe[3.1.2]
http://localhost/3.1.3/ [302] SilverStripe[3.1.3]
http://localhost/3.1.4/ [302] SilverStripe[3.1.4,3.1.5]
http://localhost/3.1.5/ [302] SilverStripe[3.1.4,3.1.5]
</pre>