change: invalidate addresses that start with a period

According to RFC 3696 (https://tools.ietf.org/html/rfc3696):
> Without quotes, local-parts may consist of any combination of
> alphabetic characters, digits, or any of the special characters
> ! # $ % & ' * + - / = ? ^ _ ` . { | } ~
> period (".") may also appear, but may not be used to start or end the
> local part, nor may two or more consecutive periods appear. Stated
> differently, any ASCII graphic (printing) character other than the
> at-sign ("@"), backslash, double quote, comma, or square brackets may
> appear without quoting. If any of that list of excluded characters
> are to appear, they must be quoted.

lib/valid_email2/address.rb

@@ -46,7 +46,8 @@ def valid?
             !domain.start_with?('-') &&
             !domain.include?('-.') &&
             !address.local.include?('..') &&
-            !address.local.end_with?('.')
+            !address.local.end_with?('.') &&
+            !address.local.start_with?('.')
         else
           false
         end

spec/valid_email2_spec.rb

@@ -92,6 +92,11 @@ class TestUserMultiple < TestModel
       expect(user.valid?).to be_falsey
     end
 
+    it "is invalid if the address starts with a dot" do
+      user = TestUser.new(email: "[email protected]")
+      expect(user.valid?).to be_falsey
+    end
+
     it "is invalid if the email contains emoticons" do
       user = TestUser.new(email: "foo🙈@gmail.com")
       expect(user.valid?).to be_falsy