Fix JSON parsing problem with Pig schema

Changes:
* if the JSON parsing exception occurs, rescue the exception
* then, expecting the format of the hive_result_schema to be
  an array of arrays, eval the variable to transform it in
  an actual Ruby array
* finally parse the array and substitute the 'nil' column
  names with '_colX' where X is the column index. This
  specific format is done to mimic what Hive does out of the
  box for anonymous column names
* if everything else fails, raise the JSON parse error exception

Fixes CLT-258
Remove unwanted change from previous commit
Merge pull request #14 from treasure-data/CLT-258

Temporary fix for the JSON parsing problem with Pig query result schema