Use latest-incoming version of Emscripten SDK

By advice from @kripken
emscripten-core/emscripten#8045 (comment)

Author: abitrolly

.travis.yml

@@ -20,8 +20,8 @@ matrix:
         - rustup component add rustfmt
         # install emsdk
         - git clone https://github.com/emscripten-core/emsdk.git
-        - ./emsdk/emsdk install sdk-incoming-64bit
-        - ./emsdk/emsdk activate sdk-incoming-64bit
+        - ./emsdk/emsdk install latest-incoming
+        - ./emsdk/emsdk activate latest-incoming
         - rustup target add asmjs-unknown-emscripten
       script:
         # the useful way to run rustfmt