Fix regex encoding: don't assume Windows-31J encoding

[mttbernardini]

As documented, using the regex flag s implies that the source string encoding is assumed to be Windows-31J. This prevents ronn from compiling valid UTF-8 sources (which actually use Unicode characters and aren't just plain ASCII), reporting the following exception:

/var/lib/gems/2.3.0/gems/ronn-ng-0.8.2/lib/ronn/roff.rb:354:in `gsub': incompatible encoding regexp match (Windows-31J regexp with UTF-8 string) (Encoding::CompatibilityError)

This PR fixes the issue by removing the unneeded flag, which was probably introduced by accident in 3dca20c#diff-8c2a2b56f00218c211ba4fc3ed93077cR358

I hope this helps, let me know.

[apjanke]

Good catch. Yes, the /s flag was introduced by mistake; I thought it had the same effect as in Perl, where it treats a multi-line regexp as a single string.

Merging. Thank you!