No more duplicated events from TracePoint API

Dupplicated events have been recently removed from the TracePoint API,
so we no longer have to deal with them. This removes the following
commands from Byebug:
* `step+`, `step-`, `next+`, `next-`.
* `set/show linetrace_plus`, `set/show forcestep`.
Merge branch 'remove_handling_of_dupped_events'
use container-based travis infrastructure
Test against 2.2