exists? has been deprecated in ruby

Replace File.exists? by File.exist?

bibjekyll.rb

@@ -80,7 +80,7 @@ def render(context)
 
           # Fix back links in the html formatted bib file
           # generated by bibtex2html
-          if File.exists?(bibhtml)
+          if File.exist?(bibhtml)
               # Read html formatted bib file
               content_bibhtml = IO.read(bibhtml)
               # determine the name of the file we are generating