Use babel-register script when running babel smoke tests

[existentialism]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	yes/no
Fixed tickets
License	MIT

When running the babel smoke test, the recent change to use --compilers js:babel-register is causing test failures as it's attempting to use babylon's babelrc:

TypeError: [BABEL] /home/travis/build/babel/babylon/node_modules/istanbul-lib-instrument/dist/index.js: Plugin 0 specified in "/home/travis/build/babel/babylon/.babelrc.env.test" was expected to return a function but returned "undefined" (While processing preset: "/home/travis/build/babel/babylon/node_modules/babel-preset-stage-0/lib/index.js") (While processing preset: "/home/travis/build/babel/babylon/node_modules/babel-preset-es2015/lib/index.js")

[codecov]

Codecov Report

Merging #442 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files          20       20           
  Lines        3504     3504           
  Branches      927      927           
=======================================
  Hits         3443     3443           
  Misses         22       22           
  Partials       39       39

Flag	Coverage Δ
#babel	82.02% <ø> (ø)	⬆️
#babylon	97% <ø> (ø)	⬆️

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