Mark gem as not loaded when spec not found

Signed-off-by: NMichael Koziarski <michael@koziarski.com>

railties/lib/rails/gem_dependency.rb

@@ -108,6 +108,8 @@ def loaded?
       @loaded ||= begin
         if vendor_rails?
           true
+        elsif specification.nil?
+          false
         else
           # check if the gem is loaded by inspecting $"
           # specification.files lists all the files contained in the gem