FIX: Undefined variable in TreeDropdownField.php

I'm guessing this was just a simple typo. Checked this by searching on SiteTree, behaves as expected after this change.
Merge pull request #2476 from kinglozzer/treedropdownfield-undefined-var

FIX: Undefined variable in TreeDropdownField.php
BUG Issue with login form failing to login in certain situations. Fixes issue #2424
Merge pull request #2480 from tractorcow/pulls/3.1-dologin-security-fix

BUG Issue with login form failing to login in certain situations. Fixes ...
FIX: Make CreditCardField use template
Merge pull request #2468 from nedmas/make-creditcardfield-use-template

FIX: Make CreditCardField use template
Allow setting of specific form actions that do not require validation

Move validation exemptions into CMSForm

Also fix buttonClicked() to skip CompositeField