Typos in Checklist model

[ilkka-rautiainen]

The array keys should be 'pos' and not 'position'.

php-trello-api/lib/Trello/Model/Checklist.php

Line 205 in b1205d9

$this->data['checkItems'][$key]['position'] = $position;

php-trello-api/lib/Trello/Model/Checklist.php

Line 211 in b1205d9

'position' => $position,

These make item position setting not work.

[cdaguerre]

@ilkka-rautiainen thanks for reporting! Could you make a pr?