Update on DoubleNagation output style

[norm-ideal]

Updated the DoubleNegation output message that was a direct output of rule variable.

Former output:

二重否定がみつかりました "ExpressionRule{elements=[TokenElement{surface='ない', tags=[], offset=0, reading='ない'}, TokenElement{surface='こと', tags=[], offset=0, reading='こと'}, TokenElement{surface='も', tags=[], offset=0, reading='も'}, TokenElement{surface='ない', tags=[], offset=0, reading='ない'}]}"

New output:

二重否定がみつかりました。 "ないこともない"

[coveralls]

Coverage increased (+0.006%) to 91.407% when pulling cf5336a on norm-ideal:cleanup into 81fc86b on redpen-cc:master.

[takahi-i]

@norm-ideal Deeply sorry for the late response. Thank you very much for the valuable improvement as always!