Update installation instructions and fix broken URL

Jim pointed out that the installation instructions are a bit out of
date.  Update them.

He also noticed that mozilla made the emscript compiler we were using
private.  Rehost it on frc971.org.

Change-Id: I03722a1ea82cbdc0a8cb6bbcc8d1d96eaf3c3a35
diff --git a/WORKSPACE b/WORKSPACE
index 3eacd10..b6b51eb 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -485,7 +485,7 @@
 """,
     sha256 = "a0c2f2c5a897577f40af0fdf68dcf3cf65557ff20c081df26678c066a4fed4b1",
     strip_prefix = "emscripten-llvm-e" + emscripten_version,
-    url = "https://s3.amazonaws.com/mozilla-games/emscripten/packages/llvm/tag/linux_64bit/emscripten-llvm-e" + emscripten_version + ".tar.gz",
+    url = "http://www.frc971.org/Build-Dependencies/emscripten-llvm-e" + emscripten_version + ".tar.gz",
 )
 
 # Fetch our Bazel dependencies that aren't distributed on npm