browser-asmjs: Update to depend on existing image from "trzeci/emscripten"

This commit updates the "browser-asmjs" images to avoid systematic
rebuild of emscripten sdk. Instead, it introduces dependency on the
image maintained by @trzeci

Note also:

* emscripten sdk updated from 1.36.7 to 1.36.14

* symlinking of "/bin/bash" as "/bin/sh" is reverted. This ensures other
  images including file like "common.debian" and relying on standard "sh"
  behavior will work as expected.

* workaround associated with CMakeForceCompiler has been removed from
  Dockerfile. Emscripten as been fixed.
  See https://github.com/kripken/emscripten/pull/4477

* we will revisit once official images are available.
  See https://github.com/kripken/emscripten/issues/4682

.gitignore

@@ -2,5 +2,6 @@ bin
 dockcross
 */test/
 Dockerfile
+browser-asmjs/Dockerfile
 */imagefiles/*
 !imagefiles/*