Refactor first with conditions to where + first

There's a deprecate use of first with conditions.
First as an alias for ``find(:first, *args)`` has been deprecated and removed:
- http://apidock.com/rails/ActiveRecord/Base/first/class
- http://apidock.com/rails/ActiveRecord/FinderMethods/first

This caused an exception:
```
UsersMailer#guide_welcome_to_anyroad: processed outbound mail in 79.7ms
ArgumentError: invalid value for Integer(): "{:conditions=>{:experiment_id=>\"guide_welcome_subject\""
from /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:332:in `Integer'
[2] pry(main)> wtf????
Exception: ArgumentError: invalid value for Integer(): "{:conditions=>{:experiment_id=>\"guide_welcome_subject\""
--
 0: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:332:in `Integer'
 1: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:332:in `block in sanitize_limit'
 2: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:332:in `map'
 3: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/connection_adapters/abstract/database_statements.rb:332:in `sanitize_limit'
 4: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation/query_methods.rb:844:in `build_arel'
 5: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation/query_methods.rb:830:in `arel'
 6: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation.rb:603:in `exec_queries'
 7: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation.rb:487:in `load'
 8: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation.rb:231:in `to_a'
 9: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation/finder_methods.rb:465:in `find_nth_with_limit'
10: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/relation/finder_methods.rb:130:in `first'
11: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/activerecord-4.1.0/lib/active_record/querying.rb:3:in `first'
12: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/vanity-1.9.1/lib/vanity/adapters/active_record_adapter.rb:98:in `retrieve'
13: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/vanity-1.9.1/lib/vanity/adapters/active_record_adapter.rb:237:in `ab_showing'
14: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/vanity-1.9.1/lib/vanity/experiment/ab_test.rb:140:in `choose'
15: /Users/bogdan/.rvm/gems/ruby-2.1.1@anyroad/gems/vanity-1.9.1/lib/vanity/helpers.rb:44:in `ab_test'
16: /Users/bogdan/workspace/anyroad/ar/app/mailers/users_mailer.rb:23:in `guide_welcome_to_anyroad'
```

The mailer was doing ``ab_test(:guide_welcome_subject)``.
This small fix solved it for me.