Refactor babel config

[TrySound]

Before #994 I want to tweak babel config a bit

• remove babel-plugin-react-transform which seems useless right now. redbox doesn't work and catching errors is not necessary with error boundaries.
• replaced stage-1 with stage-2 preset
• refactored .babelrc with js preset. It's now two times smaller.

@bvaughn how comments: false in babel config affect code? I saw it reduces somehow umd size about 50kb

[codecov-io]

Codecov Report

Merging #1019 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1019   +/-   ##
======================================
  Coverage    90.2%   90.2%           
======================================
  Files          59      59           
  Lines        1675    1675           
======================================
  Hits         1511    1511           
  Misses        164     164

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[bvaughn]

@bvaughn how comments: false in babel config affect code? I saw it reduces somehow umd size about 50kb

I don't understand what you're asking.

[TrySound]

Never mind, just confused a bit.

[bvaughn]

Okay 😄

I don't have any problems with this change, although I haven't had the time to do a before and after comparison between this and what's in master to make sure it doesn't cause any problems in the build.

If you've done that, then feel free to merge.