Update 3.0.6.md

Add reference to information disclosure in Versioned.php (SS-2013-006)
Merge pull request #2413 from ss23/patch-1

Update 3.0.6.md
BUG Fixing tabindex added to CreditCardField when tabindex is NULL

The tabindex increment *should* only be done if there is a tabindex
that has been set on a CreditCardField already, otherwise it breaks
the tab ordering.
Merge pull request #2434 from halkyon/cc_tabindex_fix_2

BUG Fixing tabindex added to CreditCardField when tabindex is NULL