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:

mapbox-gl-rtl-text/build.sh

Lines 19 to 29 in 0865804

	function setup_emsdk {
	download_and_extract \
	https://s3.amazonaws.com/mozilla-games/emscripten/releases/emsdk-portable.tar.gz \
	emsdk-portable.tgz \
	4fb6c391effc6a6cd5666abe28a8c7c48a1150f3
	emsdk-portable/emsdk update
	emsdk-portable/emsdk install latest
	emsdk-portable/emsdk activate latest
	source emsdk-portable/emsdk_env.sh
	}

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.