Export source maps

[martinRenou]

I'm having a warning when builgind bqplot-gl:

WARNING in ../bqplot/js/node_modules/bqscales/lib/version.js
Module Warning (from ./node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from '/home/martin/github/bqplot/js/node_modules/bqscales/lib/version.js.map' file: Error: ENOENT: no such file or directory, open '/home/martin/github/bqplot/js/node_modules/bqscales/lib/version.js.map'