Custom validators should inherit from Grape::Validations::Base

This is a small README change, i believe when you have custom validators they should inherit from `Grape::Validations::Base` and not from  `Grape::Validations::Validator`. I've extracted a small example here - https://github.com/rebelact/grape-validator-issue. When i inherit from `Grape::Validations::Validator` i get the following error:

````ruby
~/grape-validation-issue/config.ru:7:in `<class:API>': uninitialized constant Grape::Validations::Validator (NameError)
  from ~/grape-validation-issue/config.ru:4:in `block in <main>'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'
  from ~/grape-validation-issue/config.ru:in `new'
  from ~/grape-validation-issue/config.ru:in `<main>'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/builder.rb:49:in `eval'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/builder.rb:49:in `new_from_string'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/builder.rb:40:in `parse_file'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/server.rb:277:in `build_app_and_options_from_config'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/server.rb:199:in `app'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/server.rb:314:in `wrapped_app'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/server.rb:250:in `start'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/lib/rack/server.rb:141:in `start'
  from ~/.rvm/gems/ruby-2.1.1/gems/rack-1.5.2/bin/rackup:4:in `<top (required)>'
  from ~/.rvm/gems/ruby-2.1.1/bin/rackup:23:in `load'
  from ~/.rvm/gems/ruby-2.1.1/bin/rackup:23:in `<main>'
  from ~/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `eval'
  from ~/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `<main>'
````

When i inherit from `Grape::Validations::Base` it works as expected.