Only remove the first iteration of backtrace_remove

[vprigent]

Our work setup is as follows:
We use docker for our rails app
We store our app within "/portal" in docker.

We also have a few folders called ".../portal/..." within the code which were getting swallowed by gsub.

backtrace_remove defaults to "#{Rails.root}/" and the only other search result for it within Github returned a similar usage but for a gem

[kbrock]

• This pretty much assumes that the backtrace_remove is on the left hand side of the file. (think this is a valid assumption)
• Wonder if a regexp for backtrace_remove would meet your needs.
• This may be a breaking change for people.

Having said all that, I think this is a great addition.