README.webp_js: update note about emscripten version
README.webp_js: update note about emscripten version

keep the minimum, but recommend the latest so we don't need to update
this in the future.

BUG=webp:463

Change-Id: I0615039323d3c77684c6be2e890514f6973cf535

webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp