Improve regexp in Exception#backtrace spec

Author: eregon

spec/ruby/core/exception/backtrace_spec.rb

@@ -43,7 +43,7 @@
       # This regexp is deliberately imprecise to account for the need to abstract out
       # the paths of the included mspec files and the desire to avoid specifying in any
       # detail what the in `...' portion looks like.
-      line.should =~ /^[^ ]+\:\d+(:in `[^`]+')?$/
+      line.should =~ /^.+:\d+:in `[^`]+'$/
     end
   end