Use json_pure.
Use json_pure.

While this installs json_pure on all Ruby versions, it will be used only
on Ruby 1.8.7. Since json_pure doesn't compile any C extensions, it
makes SequenceServer installation a little better.

close #197

Signed-off-by: Anurag Priyam <6e6ab5ea9cb59fe7c35d2a1fc74443577eb60635@gmail.com>

elsiklab/sequenceserver,yeban/sequenceserver,IsmailM/sequenceserver,yeban/sequenceserver,IsmailM/sequenceserver,IsmailM/sequenceserver,yeban/sequenceserver,yeban/sequenceserver,elsiklab/sequenceserver,elsiklab/sequenceserver