Update xo to version 0.12.0 🚀

[greenkeeperio-bot]

Hello :wave:

:rocket::rocket::rocket:

xo just published its new version 0.12.0, which is not covered by your current version range.

If this pull request passes your tests you can publish your software with the latest version of xo – otherwise use this branch to work on adaptions and fixes.

Happy fixing and merging :palm_tree:

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The new version differs by 9 commits .

• 5ddc8f7 0.12.0
• 852e2dc simplify CLI tests
• f2c88be readme tweaks
• ded7b07 always use the Babel parser
• ac673fc bump dependencies
• 356befc Merge pull request #55 from vdemedes/open-files
• 29fa0c1 open files with issues in an editor
• 0a5060c Close #54 PR: Fixes the tests to be synchronous and not use t.end()..
• 3ec8aa1 fix travis

See the full diff.

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