Fix a couple issues on ruby 1.8
Fix a couple issues on ruby 1.8

mime-types 2 does not support ruby 1.8, so use mime-types < 2 on
ruby 1.8.  Adding json is not necessary in the gemfile, since it
is specified in the gemspec.

kematzy/roda,jeremyevans/roda,Freaky/roda,kematzy/roda,jeremyevans/roda,johan--/roda,Freaky/roda,celsworth/roda,duykhoa/roda,Freaky/roda,johan--/roda,duykhoa/roda,janko-m/roda,mikz/roda,mikz/roda,duykhoa/roda,janko-m/roda,janko-m/roda,johan--/roda,celsworth/roda,kematzy/roda,mikz/roda,celsworth/roda,jeremyevans/roda