Bugfixes for the new ROOT version

- compiled the new ROOT version (5.34.18) using the 64bit virtual machine, in order to be used via travis;
- added it to the travis matrix in .travis.yml
- our current code shouldn't pass the code integration test as in the new root version [TH2F.GetBinLowEdge](https://github.com/BristolTopGroup/DailyPythonScripts/blob/master/src/cross_section_measurement/00_pick_bins.py#L155) method exits with error (which makes sense, as the axis needs to be specified);
- in the next commit, this will be fixed.