chore: fix issues with nyc and mocha

[nfischer]

This fixes 2 issues, both of which are vaguely related to babel transpiling during tests:

• The new mocha version uses the --require flag instead of --compilers.
• nyc 12.0.0 causes mysterious "Transformation errors" with transpiling during tests.

This PR addresses the first issue by switching the flag as required. This PR addresses the second issue by downgrading nyc, as v11 is the last release which supports node v4.

Fixes #144

[codecov-io]

Codecov Report

Merging #145 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #145   +/-   ##
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  Coverage     100%   100%           
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  Files           4      4           
  Lines          75     75           
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  Hits           75     75

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