Merge upstream libdivecomputer changes from Jef Driesen.
* https://github.com/libdivecomputer/libdivecomputer:
Add Travis CI integration
Fix the transport command-line parameter
Document dc_descriptor_get_model
Include stddef.h in iostream.h
Add support for the Mares Smart Air
Fix the average depth for older OSTC dives
Add support for the Oceanic Pro Plus X