Remove /gems/ from default backtrace exclusion patterns.

As discussed in #1536 and #1554, the failure can often be in a gem.

We may add an additional gem filtering API later on but for
now I just want to try this on a few projects and see how the
experience is.
Merge pull request #1641 from rspec/tweak-backtrace-formatter

Remove /gems/ from default backtrace exclusion patterns.