Fix nullish coalescing when the RHS is an object literal

[alangpierce]

Fixes #505

The RHS is meant to be transpiled into an expression arrow function, but if the
expression is an object literal, it gets interpreted as a block body. To fix, we
can unconditionally surround the arrow body in parens. This is a little ugly in
that it leaves an extra space, but avoiding that is probably more trouble than
it's worth.

Also fix two issues with test infra:

• The build was only running the very last build configuration due to what seems to be a
  breaking change in Travis. Updated the yaml file to use jobs instead of matrix, which
  seems to work better.
• Three Babel tests had broken due to fixture tests that were testing unpinned behavior (looks
  like regex transformation and preset-env behavior). I was hoping to fix them by pinning the
  dependencies or updating to latest Babel, but ended up just updating the expected test code
  for now. I confirmed that this was a regression on the Babel end rather than anything caused by
  a Sucrase change.

[codecov-io]

Codecov Report

Merging #516 into master will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
- Coverage   82.71%   82.69%   -0.02%     
==========================================
  Files          55       55              
  Lines        5617     5618       +1     
  Branches     1263     1264       +1     
==========================================
  Hits         4646     4646              
  Misses        681      681              
- Partials      290      291       +1     

Impacted Files	Coverage Δ
src/transformers/ReactHotLoaderTransformer.ts	94.73% <50.00%> (-5.27%)	⬇️
src/TokenProcessor.ts	92.72% <100.00%> (ø)
...transformers/OptionalChainingNullishTransformer.ts	84.93% <100.00%> (ø)

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