switch to my own custom-build root_v5.34.18

root does not provide a binary package for the ubuntu LTS used on
travis-ci.  We used to rely on the package built by the rootpy folks,
which they have now dropped (it probably was some old version...).
Anyway, at some point I needed some features that they didn't enable,
so I built my own. Instructions to run the build process are at
gerbaudo/SusyNtuple/scripts/build_root.sh