Add basic test (#22)

.babelrc

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+{
+  "presets": ["react-native"]
+}

__tests__/TimeAgo.test.js

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+import React from 'react'
+import { Text } from 'react-native'
+import renderer from 'react-test-renderer'
+import moment from 'moment'
+import TimeAgo from '../TimeAgo'
+
+describe('TimeAgo', () => {
+  test('Renders', () => {
+    const timestamp = moment().subtract(10, 'days')
+    const component = renderer.create(
+      <TimeAgo time={timestamp} />
+    )
+    const instance = component.root
+    expect(instance.findByType(Text).props.children).toEqual('10 days ago')
+  })
+})