Fix stacktrace conversion

[sdzyba]

I faced an issue with stacktrace first entry being incorrect.
Say in application we have something like that:

gitlab.com/project/api/repositories.(*ModelTranslation).Create
 	/service/repositories/model_translation.go:33
gitlab.com/project/api/services.(*Model).Create.func1
 	/service/services/model.go:68
github.com/go-pg/pg.(*Tx).RunInTransaction
 	/go/pkg/mod/github.com/go-pg/pg@v8.0.4+incompatible/tx.go:79
...

But Sentry received an exception with a bit different trace:

  File "/go/pkg/mod/github.com/pkg/errors@v0.8.1/errors.go", line 151, in WithStack
    callers(),
  File "/service/services/model.go", line 68, in func1
    err = modelTranslationRepo.Create(txCtx, mt)
  File "/go/pkg/mod/github.com/go-pg/pg@v8.0.4+incompatible/tx.go", line 79, in RunInTransaction
    if err := fn(tx); err != nil {
...

So the first entry in Sentry's trace leads to pkg/errors, but originally it was referring to the application code.
While trying to find the reason behind that I noticed stacktrace conversion in raven-go looked almost the same as in logrus_sentry except for a - 1 part here.
To be honest I'm not sure how exactly it works, but I did the same change for logrus_sentry and now I get correct stacktrace and it seems like all tests here still pass.

[klaus2Dancer]

i try. but it didn't work