apm: add support for github.com/pkg/errors@master

[axw]

github.com/pkg/errors has broken backwards compatibility
on master, which will become 0.9.0. Until we can get rid
of support for the older version (which, undoubtedly, many
people are using), we will support both by using reflection.

Bump version to v1.1.3.

Fixes #408

[codecov-io]

Codecov Report

Merging #409 into master will decrease coverage by 0.43%.
The diff coverage is 60%.

@@            Coverage Diff            @@
##           master    #409      +/-   ##
=========================================
- Coverage   84.33%   83.9%   -0.44%     
=========================================
  Files         103     102       -1     
  Lines        6467    6132     -335     
=========================================
- Hits         5454    5145     -309     
+ Misses        945     917      -28     
- Partials       68      70       +2

Impacted Files	Coverage Δ
error.go	75.97% <60%> (-6.38%)	⬇️
stacktrace/context.go	77.5% <0%> (-2.5%)	⬇️
module/apmsql/pq/parser.go	78.78% <0%> (-2.03%)	⬇️
stacktrace/library.go	100% <0%> (ø)	⬆️
gocontext.go	100% <0%> (ø)	⬆️
module/apmot/internal/harness/api_checkers.go

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