Fix a few warnings that appear in ruby 2.5

lib/configatron/integrations/rails.rb

@@ -42,7 +42,7 @@ def self.init(root = nil, env = nil)
       config_files.collect! {|config| File.expand_path(config)}.uniq!
 
       config_files.each do |config|
-        if File.exists?(config)
+        if File.exist?(config)
           # puts "Configuration: #{config}"
           require config
         end

lib/configatron/proc.rb

@@ -11,7 +11,7 @@ def initialize(&block)
     end
 
     def call
-      unless @val
+      unless defined? @val
         val = self.block.call
         self.execution_count += 1
         if finalize?

test/functional/configatron.rb

@@ -159,7 +159,7 @@ class Critic::Functional::ConfigatronTest < Critic::Functional::Test
   describe 'nil value' do
     it 'remembers a nil value' do
       @kernel.a = nil
-      assert_equal(nil, @kernel.a)
+      assert_nil @kernel.a
     end
   end
 end