initalize chgColumns as object to avoid error when table columns have…

[willgriffin]

… the same name as Array object properties

We have a table with a "length" field that was resulting in

[ERROR] RangeError: Invalid array length
    at columns (/home/will/Projects/node-ultimate-migrate/lib/builder.js:695:54)
    at ResultEmitter.<anonymous> (/home/will/Projects/node-ultimate-migrate/node_modules/umigrate-mariadb/index.js:209:29)
    at emitNone (events.js:106:13)
    at ResultEmitter.emit (events.js:208:7)
    at ResultEmitter._complete (/home/will/Projects/node-ultimate-migrate/node_modules/mariasql/lib/Client.js:699:12)
    at QueryStream.<anonymous> (/home/will/Projects/node-ultimate-migrate/node_modules/mariasql/lib/Client.js:682:10)
    at emitNone (events.js:106:13)
    at QueryStream.emit (events.js:208:7)
    at endReadableNT (_stream_readable.js:1064:12)
    at _combinedTickCallback (internal/process/next_tick.js:139:11)

this fixes it, thanks for the lib <3