Fix notice on atomic field

[inigoflores]

#157 introduced a bug that triggers multiple Notice messages:

Notice (1024): SoftDeleteBehavior::setup(): model PropertyValue has no field!atomic [APP/Plugin/Utils/Model/Behavior/SoftDeleteBehavior.php, line 74]
Notice (1024): SoftDeleteBehavior::setup(): model EmailAddress has no field!atomic [APP/Plugin/Utils/Model/Behavior/SoftDeleteBehavior.php, line 74]
Notice (1024): SoftDeleteBehavior::setup(): model Property has no field!atomic [APP/Plugin/Utils/Model/Behavior/SoftDeleteBehavior.php, line 74]
Notice (1024): SoftDeleteBehavior::setup(): model Contact has no field!atomic [APP/Plugin/Utils/Model/Behavior/SoftDeleteBehavior.php, line 74]

It is easily fixed by removing:

 'atomic' => true,

from the $default array declaration.

This is not needed as, a default value is already set by:

protected $_atomic = true;