Support 23.0

[kostis]

Summary

This is the first commit, just to try tests on Travis and see where we are.
Besides supporting 23.0, we take out support for versions prior to 19.0
and update Travis to use xenial (Ubuntu 16.04).

[aronisstav]

Took me a while to remember why the old instrumenter needed all the tricks it had (I added one more comment to help me remember sooner next time, see 46d5f4f)...

Tests are not yet finished, and tests-real are breaking on a stacktrace-related test (maybe #315 is related), although the actual reason of the failure is currently the lack of support for the persistent_term module...

Still, this (still really ugly) version might be useful to someone...

[aronisstav]

New pain: the main tests (which are cover-enabled) are now taking too long. Unfortunately there is a LOT of redundancy there:

• Concuerror is started from the shell every time so...
• ... Erlang is started every time and...
• ... Concuerror is cover-compiled every time

Still, there shouldn't be such a huge increase in time between 22.3 and 23.0...

[codecov]

Codecov Report

Merging #311 into master will decrease coverage by 0.28%.
The diff coverage is 91.25%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
- Coverage   95.76%   95.47%   -0.29%     
==========================================
  Files          12       12              
  Lines        3682     3738      +56     
==========================================
+ Hits         3526     3569      +43     
- Misses        156      169      +13     

Flag	Coverage Δ
#tests	?
#tests-1	77.01% <80.87%> (?)
#tests-2	68.91% <72.67%> (?)
#tests_real	84.77% <83.60%> (-0.02%)	⬇️
#unit_tests	2.65% <0.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
src/concuerror_estimator.erl	95.04% <ø> (ø)
src/concuerror_logger.erl	96.65% <ø> (ø)
src/concuerror_options.erl	98.82% <ø> (ø)
src/concuerror_scheduler.erl	97.13% <ø> (ø)
src/concuerror_window_average.erl	100.00% <ø> (ø)
src/concuerror_dependencies.erl	87.73% <25.00%> (-1.62%)	⬇️
src/concuerror_callback.erl	94.80% <88.15%> (-0.66%)	⬇️
src/concuerror_instrumenter.erl	99.01% <98.88%> (+2.89%)	⬆️
src/concuerror_loader.erl	93.82% <100.00%> (ø)

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