Fix a few warnings that appear in ruby 2.5

[kreynolds]

Fixed a couple of small warnings that appear when running Configatron with ruby 2.5

lib/configatron/proc.rb:14: warning: instance variable @val not initialized

DEPRECATED: Use assert_nil if expecting nil from /Users/kreynolds/Projects/configatron/test/functional/configatron.rb:162. This will fail in Minitest 6.