Change how external thrift files' generated code is required

Currently, Ruby generated structs assume that any external Thrift files' generated code can be found in exactly the same directory, which is often not the case. This leads to issues when you have a bunch of different thrift files and generated code living in different places.

The best solution seems to be to change the generator to omit the path leading up to the included thrift file's name, and just assuming that the developer will put the external generated code into the Ruby load path.