Update arrow from 0.15.6 to 0.15.8 (#1591)
Update arrow from 0.15.6 to 0.15.8 (#1591)

Co-authored-by: Fábio Rosado <504d498c4e6912c88badf931727242f17b94f32f@gmail.com>
jacobtomlinson/opsdroid,FabioRosado/opsdroid,opsdroid/opsdroid