chore: fix issues with nyc and mocha (#145)

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