fix tracer deprecation warnings (#112)

Author: diegofigueroa

lib/graphql/fragment_cache.rb

@@ -33,12 +33,11 @@ class << self
       def use(schema_defn, options = {})
         verify_interpreter_and_analysis!(schema_defn)
 
-        schema_defn.tracer(Schema::Tracer)
-
         if GraphRubyVersion.after_2_2_5?
           schema_defn.trace_with(GraphQL::Tracing::LegacyHooksTrace)
           schema_defn.instance_exec { own_instrumenters[:query] << Schema::Instrumentation }
         else
+          schema_defn.tracer(Schema::Tracer)
           schema_defn.instrument(:query, Schema::Instrumentation)
         end