Migration complete and internal release 1.9

Migration from BitBucket to GitHub has completed! In particular, the repository for the development version of the library becomes

https://github.com/ariadne-cps/ariadne.

This renaming is in preparation for the 2.0 official release of Ariadne, tentatively due at the beginning of 2019. The migration also comes with a new 1.9 internal release, where we improved stability and increased OS and compiler compatibility. We introduced continuous integration and code coverage to our development flow, with the purpose of driving improvements to code quality.