Error for presetColors objects in Sketch
I am getting a warning for Sketch when using presetColors object as described:
```
bundle.js:3249 Warning: Failed prop type: Invalid prop `presetColors[1]` of type `object` supplied to `ColorPicker`, expected `string`.
    in ColorPicker (created by FormatDialog)
    in FormatDialog (created by ConfigTable)
    in div (created by ConfigTable)
    in ConfigTable (created by SpotfireTable)
    in SpotfireTable
printWarning @ bundle.js:3249
warning @ bundle.js:3273
checkReactTypeSpec @ bundle.js:59741
validatePropTypes @ bundle.js:38081
createElement @ bundle.js:38135
render @ bundle.js:70738
(anonymous) @ bundle.js:62361
measureLifeCyclePerf @ bundle.js:61641
_renderValidatedComponentWithoutOwnerOrContext @ bundle.js:62360
_renderValidatedComponent @ bundle.js:62387
_updateRenderedComponent @ bundle.js:62311
_performComponentUpdate @ bundle.js:62289
updateComponent @ bundle.js:62210
performUpdateIfNecessary @ bundle.js:62126
performUpdateIfNecessary @ bundle.js:17143
runBatchedUpdates @ bundle.js:10590
perform @ bundle.js:20484
perform @ bundle.js:20484
perform @ bundle.js:10529
flushBatchedUpdates @ bundle.js:10612
closeAll @ bundle.js:20550
perform @ bundle.js:20497
batchedUpdates @ bundle.js:63452
batchedUpdates @ bundle.js:10537
dispatchEvent @ bundle.js:63610
bundle.js:3249 Warning: Failed prop type: Invalid prop `presetColors[1]` of type `object` supplied to `Sketch`, expected `string`.
    in Sketch (created by ColorPicker)
    in ColorPicker (created by FormatDialog)
    in div (created by FormatDialog)
    in div (created by FormatDialog)
    in FormatDialog (created by ConfigTable)
    in div (created by ConfigTable)
    in ConfigTable (created by SpotfireTable)
    in SpotfireTable
```
Calling:
```
<SketchPicker 
                        color={ this.state.color } 
                        disableAlpha = {true}
                        presetColors = {[ "#FFFFFF",
                            {color:'#DA9694', title:'Red'},
                            {color:'#C4D79B', title:'Green'},
                            {color:'#D7D7D7', title:'Gray'},
                            {color:'#1F497D', title:'Header'},
                            {color:'#C5E5F5', title:'SubHeader'},
                            "#000000"
                        ]}
                        onChange={ this.handleColor.bind(this) } 
                        name={this.state.name}/>
```