fix(VictoryGroup): addition on Dates without getTime fails depending on runtime
fix(VictoryGroup): addition on Dates without getTime fails depending on runtime

Apparently using a constructor with the plus syntax (or invalid values that eventually get into this
code path) result in VictoryGroup being unable to render children properly and displaying empty
graphs. This change invokes `getTime()` first after the `instanceof Date` check which should always
succeed.

See: FormidableLabs/victory-native#128, FormidableLabs/victory#728 and FormidableLabs/victory#438

FormidableLabs/victory-chart,FormidableLabs/victory-chart