Attempt to modify frozen string litterals

[MadBomber]

I just ran into a problem in the to_s method of Slop::Options. The lines where where << is used on the local variable 'str' are raising the error ...

/Users/dewayne/.rvm/gems/ruby-2.7.1/gems/slop-4.8.2/lib/slop/options.rb:108:in `block in to_s': can't modify frozen String: "Install stuff from the directory common_debug into the current project\n" (FrozenError)

Looks like replacing the "str <<" with "str +=" fixes it. I will work up a PR.

[MadBomber]

I forked the repo; cloned it to my workstation and ran rake test. Got about 5 failures; I created a new branch with a PR to replace the "str <<" with "str +=" in the to_s method in the options class.

After this fix the rake test produced zero errors.

The PR is #255

[leejarvis]

Moving discussion to that PR