Replace gsub! with gsub for replacement in files

gsub! returns nil when there are no matches.  As such, files with no
tokens within them have all their content cleared.  Using gsub resolves
this, as gsub will return the original contents of the files when there
are no matches.
Merge pull request #2 from ngm/fix-files-with-no-replacements

Replace gsub! with gsub for replacement in files