Drop R15 support and test R17 instead.
New format for otp release names in .travis.yml
Re-raise original exceptions from wes:command/2+3

Any exception that occurs inside the actor/channel during a call to
wes:command/2+3 will now be re-raised to the caller. In addition, the
stats module will receive a stop event with the argument {exception,
Class, Reason, Stacktrace}.
Merge branch 'eproxus-expose-errors'