Be more explicit about sdk version

Error: No tool or SDK found by name '1.38.21'.

Author: abitrolly

.travis.yml

@@ -17,7 +17,7 @@ matrix:
 
     - language: rust
       rust: "1.27.2"
-      env: EMSDK_VERSION=1.38.21
+      env: EMSDK_VERSION=sdk-1.38.21
       install:
         #- rustup component add rustfmt
         # install emsdk