Run Emscripten build in CI

[isaacbrodsky]

Using the Docker images described in http://kripken.github.io/emscripten-site/docs/compiling/Travis.html

Fixes #5

[isaacbrodsky]

Unless Emscripten is being run at the wrong part of the build process, I'm not sure why this is failing. It is indeed running Emscripten, and apparently the right version, but it causes git diff to believe that the libh3.js file changes.

edit: after trying on another Linux system, it seems like maybe the artifact built on Linux is different than the artifact built on Mac OSX. Not sure how we want to handle that in CI.

[nrabinowitz]

Hm. I'll need to look into this. One option is to change the build to always use a docker instance, even locally, which might be helpful for consistency (no longer relying on local env).

[nrabinowitz]

Closing, covered by #14