Exclude exe/rspec from backtraces.

When users had installed rspec-core as a bundler :git dependency,
the rspec binary (locally defined in `exe/rspec`) would be included
in backtraces. This wouldn't affect users who had done a normal
gem install since rubygems copies it to a `bin` directory.

Fixes #1646.
Only ignore JRuby files on JRuby.

This should make things slightly faster on MRI as it’s
one less regex to check.
Remove unused file.
Update changelog.
Merge pull request #1648 from rspec/remove-exe-rspec-from-backtrace

Remove exe rspec from backtrace