* Add iOS SDK build and test to rust 1.25.0 version of travis CI build
* Address review comments
* Move iOS testing and document generation into post test jobs
Principally, this adds producing libmentat_ffi.a for consumption by Carthage.
To achieve this, we disable bitcode and bump the miniumum iOS version
to 11. In addition, we make things open and public so that consumers
can, well, consume.