Handle diamond inheritance, treat derived attributes correctly

* Closes issue #240 ( **diamond inheritance should never require hand-editing code** )
  - This was embarrassingly easy once I figured out what needed done - I just removed the depth limiting code, going from an array of `int` to a `bool` and inverting its default value
  - In spite of not being able to find a patch from @rbertucat that made diamond inheritance work, it turns out that he had modified STEPattributeList::push() to reject duplicates - which is necessary for diamond inheritance

* Closes issue #232 ( **missing asterisk for derived attribute / data lost** )
  - when marking an attribute as derived, search for one with the correct owning entity
  - solves a problem when two inherited attributes have the same name and only one is derived; the other was incorrectly marked as derived

**Minor changes**
* cleanup in fedex_plus source and in generated code
* don't run gcc for travis-ci, since clang is faster

test results on [cdash](http://my.cdash.org/viewTest.php?buildid=508473) and (soon) travis-ci