Fix require cycle in TransitionItem.js

[Jonathan0wh]

Minor fix for #175. This should fix the require cycle warning below when installing the package, by bypassing Utils/index.js:

require.js:114 Require cycle: 
node_modules/react-navigation-fluid-transitions/TransitionItem.js 
-> node_modules/react-navigation-fluid-transitions/Utils/index.js 
-> node_modules/react-navigation-fluid-transitions/Utils/getResolvedMetrics.js 
-> node_modules/react-navigation-fluid-transitions/TransitionItem.js

Require cycles are allowed, but can result in uninitialized values. 
Consider refactoring to remove the need for a cycle.

[kaueDM]

Can someone merge it? Is really annoying this warning in the console.

@chrfalch