Respect TypeModifierOmitStackTrace in Builder

Previous implementation of `WithCause` method didn't respect the
`TypeModifierOmitStackTrace` modifier. A stack trace was erroneously
collected if the error that was passed to `WithCause` didn't have
stack trace.

I had to upgrade `github.com/stretchr/testify` to a slightly more modern
version to be able to use `Same` assertion.

fix unwrap on typed nil (#42)

Merge joomcode/unstable_master

Merge feature/gomod

Merge go-1.8-fix

Remove redundant `.Transparent()` calls

Merge feature/panic-stack-trace-chained

Merge pull request #15 from isopov/common-fmt

gofmt common.go after adding comments

Merge pull request #15 from isopov/common-fmt

gofmt common.go after adding comments

godoc