Upgrade Codecov to 2.1.4

[jimmylai]

Summary

Codecov reports upload stop working for a while. Upgrade the version to see if the issue can be fixed.
https://codecov.io/gh/Instagram/LibCST/pulls

Test Plan

After CI builds, Codecov commented on PR again.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@ed1a2ad). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #311   +/-   ##
=========================================
  Coverage          ?   93.96%           
=========================================
  Files             ?      222           
  Lines             ?    21412           
  Branches          ?        0           
=========================================
  Hits              ?    20119           
  Misses            ?     1293           
  Partials          ?        0           

Impacted Files	Coverage Δ
libcst/codemod/commands/ensure_import_present.py	78.57% <0.00%> (ø)
libcst/_version.py	100.00% <0.00%> (ø)
libcst/codegen/gather.py	96.38% <0.00%> (ø)
libcst/codemod/_context.py	100.00% <0.00%> (ø)
libcst/codemod/_visitor.py	82.85% <0.00%> (ø)
libcst/_parser/parso/utils.py	85.56% <0.00%> (ø)
libcst/metadata/__init__.py	100.00% <0.00%> (ø)
libcst/matchers/tests/test_decorators.py	100.00% <0.00%> (ø)
libcst/_batched_visitor.py	94.54% <0.00%> (ø)
libcst/_parser/types/production.py	100.00% <0.00%> (ø)
... and 212 more

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[jimmylai]

@josieesh , after this fix is merged, you can also take look at the coverage report on your #308 to see if some code can be covered by adding more test cases.