Fix WarningsSpy to capture more lines

If you had a bunch of lines that looked like this:

    /path/to/some/file:13:in `some_method': Some error (SomeException)
    Another line for the error
        /path/to/file1.rb:44:in `some_method'
        /path/to/file2.rb:29:in `some_other_method'

The first line would not be captured and re-send to standard out. This
commit fixes that.
Fix test failures on other Rubies