Explicitly name classes for AR associations

Fixes 'Vanity is not missing constant VanityConversion' errors

lib/vanity/adapters/active_record_adapter.rb

@@ -22,7 +22,7 @@ class VanitySchema < VanityRecord
       # Metric model
       class VanityMetric < VanityRecord
         set_table_name :vanity_metrics
-        has_many :vanity_metric_values
+        has_many :vanity_metric_values, :class_name => "Vanity::Adapters::ActiveRecordAdapter::VanityMetricValue"
 
         def self.retrieve(metric)
           find_or_create_by_metric_id(metric.to_s)
@@ -38,7 +38,7 @@ class VanityMetricValue < VanityRecord
       # Experiment model
       class VanityExperiment < VanityRecord
         set_table_name :vanity_experiments
-        has_many :vanity_conversions, :dependent => :destroy
+        has_many :vanity_conversions, :dependent => :destroy, :class_name => "Vanity::Adapters::ActiveRecordAdapter::VanityConversion"
 
         # Finds or creates the experiment
         def self.retrieve(experiment)