Fix errors in embedded sourcemaps

[richvdh]

The sourcemaps in the browserified libraries were incorrect due to the addition
of the header. We can fix this using gulp-sourcemaps to export correct
sourcemaps.

[fehguy]

@richvdh looks like the build doesn't like es6 syntax. Can you address and update the PR?

[richvdh]

Thanks for the feedback: looks like a dependency of gulp-sourcemaps (strip-bom) is using the arrow notation.

For now I've pinned gulp-sourcemaps to 1.x, which claims compatibility with node 0.10. Obviously an alternative would be to bump the version of node requested by travis.yml: 0.10 is pretty old these days [1].

[1] https://github.com/nodejs/LTS#lts-schedule