Make Nyaplot compatible with JRuby

[lokeshh]

Currently Nyaplot is not compatible with JRuby.

lokeshh:~/workspace/daru (rake_task) $ rvm use jruby
Using /home/ubuntu/.rvm/gems/jruby-1.7.19
lokeshh:~/workspace/daru (rake_task) $ gem install nyaplot
WARN: Unresolved specs during Gem::Specification.reset:
      ffi (>= 0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
ERROR:  Error installing nyaplot:
        nyaplot requires Ruby version >= 2.0.

Related to SciRuby/daru#122

Solving this would help make Daru compatible with JRuby