message: add octicon to MergeBox
diff: import React, {Fragment} from 'react'
  import classnames from 'classnames'
  +import Octicon from '@github/octicons-react'
  import MergeStatus from '../src/MergeStatus'
  import Caret from '../src/Caret'
  import Box from '../src/Box'
  @@ -18,10 +19,13 @@ export default function MergeBox({ state }) {
  <span style={{flexGrow: 0}}><MergeStatus state={state}/></span>
  <Box ml={3} border={[true, 'green']}>
  <Caret edge='left'/>
  - <Block py={2} px={4} border='bottom'>
  + <Block py={2} px={4} border='bottom' style={{display: 'flex'}}>
  + <Block m={1}><Octicon name='check' size={32} /></Block>
  + <Block display='inline'>
  <Text tag='p' m={1} fontSize={2} fontWeight='bold'>This branch has no conflicts with the base branch</Text>
  <Text tag='p' m={1} mt={0} fontSize={0}>Merging can be performed automatically</Text>
  </Block>
  + </Block>
  <Block py={2} px={4} bg='gray-light'>
  <Block py={1} pr={1} >
  <MergeButton scheme='primary' />
  