updates ruby symbol syntax in README.md

Hello,

First of all, I would like to express my gratitude to creator of this gem. It is very helpful.

I would like readme to be updated with a new symbol syntax, in my opinion it looks better. Experience developers, who maintain legacy systems will be able to read new one, but fresh rubyists will copy and paste this code without thinking. I don't care about gem source code, because users don't see it, but in readme we should see examples with updated syntax.

Thank you!

README.md

@@ -17,21 +17,21 @@ bundle install
 Then add the following to your model:
 
 ```ruby
-validates :my_email_attribute, :email => true
+validates :my_email_attribute, email: true
 ```
 
 ## Strict mode
 
 In order to have stricter validation (according to http://www.remote.org/jochen/mail/info/chars.html) enable strict mode. You can do this globally by adding the following to your Gemfile:
 
 ```ruby
-gem 'email_validator', :require => 'email_validator/strict'
+gem 'email_validator', require: 'email_validator/strict'
 ```
 
 Or you can do this in a specific `validates` call:
 
 ```ruby
-validates :my_email_attribute, :email => {:strict_mode => true}
+validates :my_email_attribute, email: {strict_mode: true}
 ```
 
 ## Validation outside a model
@@ -48,7 +48,7 @@ EmailValidator.valid?('narf@example.com') # boolean
 ### Strict mode
 
 ```ruby
-EmailValidator.regexp(:strict_mode => true)
+EmailValidator.regexp(strict_mode: true)
 ```
 
 ## Thread safety