Build against specific emscripten version

[ChrisLoer]

Right now the build script verifies the hash of the emscripten package it downloads, but after it downloads it updates to the latest version of emscripten:

https://github.com/mapbox/mapbox-gl-rtl-text/blob/08658044385df0b012eab318268be87847de3202/build.sh#L19-L29

This is a weird way of guaranteeing the version -- when a new version of emscripten is released and published to https://s3.amazonaws.com/mozilla-games/emscripten/releases/emsdk-portable.tar.gz, the build will break until the hash is updated (at which point it will be using the new version).

Instead, we should point the build to a specific/archived release.