[Resolver] Reword message

via @segiddins

lib/cocoapods/resolver.rb

@@ -196,7 +196,7 @@ module Pod
         cached_sets[name] = set
         unless set
           raise Informative, "Unable to find a specification for " \
-            "`#{dependency}` dependent by #{dependent_spec}."
+            "`#{dependency}` depended upon by #{dependent_spec}."
         end
       end
       cached_sets[name]

spec/unit/resolver_spec.rb

@@ -107,7 +107,7 @@ module Pod
           @resolver.resolve
         end.message
         message.should.match /Unable to find a specification/
-        message.should.match /`Windows` dependent by BlocksKit/
+        message.should.match /`Windows` depended upon by BlocksKit/
       end
 
       it 'does not raise if all dependencies are supported by the platform of the target definition' do