Added debugging output to debug type= problem on Travis.

ruby/Gemfile.lock

 PATH
   remote: .
   specs:
-    google-protobuf (3.0.0.alpha.4.0)
+    google-protobuf (3.0.0.alpha.5.0)
 
 GEM
   remote: https://rubygems.org/

ruby/ext/google/protobuf_c/defs.c

@@ -543,7 +543,8 @@ VALUE FieldDescriptor_name_set(VALUE _self, VALUE str) {
 
 upb_fieldtype_t ruby_to_fieldtype(VALUE type) {
   if (TYPE(type) != T_SYMBOL) {
-    rb_raise(rb_eArgError, "Expected symbol for field type.");
+    rb_raise(rb_eArgError, "Expected symbol for field type, not: %s (%d)",
+             RSTRING_PTR(rb_inspect(type)), TYPE(type));
   }
 
 #define CONVERT(upb, ruby)                                           \