Remove existing .js files on build

[guybedford]

Fixes #108.

[codecov-io]

Codecov Report

Merging #110 into master will decrease coverage by 0.69%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #110     +/-   ##
=========================================
- Coverage   76.44%   75.74%   -0.7%     
=========================================
  Files           7        7             
  Lines         433      437      +4     
=========================================
  Hits          331      331             
- Misses        102      106      +4

Impacted Files	Coverage Δ
src/cli.js	0% <0%> (ø)	⬆️

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