rb-Sisimai should work on JRuby

[azumakuniyuki]

• https://twitter.com/hiroysato/status/665097286939049984
• http://qiita.com/hiroysato/items/30c3bb07cbed825cf7a5

[azumakuniyuki]

Sisimai works on JRuby 9.0.0.0 but some warnings are displayed.
https://travis-ci.org/sisimai/rb-Sisimai/jobs/106640594

...
/home/travis/build/sisimai/rb-Sisimai/spec/sisimai/msp/private-children_spec.rb:296: warning: Dir.exists? is a deprecated name, use Dir.exist? instead
/home/travis/build/sisimai/rb-Sisimai/spec/sisimai/msp/private-children_spec.rb:296: warning: Dir.exists? is a deprecated name, use Dir.exist? instead
...
/home/travis/build/sisimai/rb-Sisimai/lib/sisimai/datetime.rb:226: warning: in the next release. Return nil in #coerce if the coercion is impossible.
..../home/travis/build/sisimai/rb-Sisimai/lib/sisimai/datetime.rb:237: warning: Numerical comparison operators will no more rescue exceptions of #coerce
...

[azumakuniyuki]

Sisimal works on JRuby 9.0.4.0 but some warning messages which are the same of above are displayed
https://travis-ci.org/sisimai/rb-Sisimai/jobs/106640595