Nvm install in .travis.yml

The one in travis.sh was failing:

    ./travis.sh: line 56: nvm: command not found
    The command "./travis.sh" exited with 127.

.travis.yml

@@ -20,5 +20,6 @@ matrix:
       env: CLIPPY=true
     - rust: nightly
       env: EMSCRIPTEN=true
+      script: nvm install 9 && ./travis.sh
 
 script: ./travis.sh

travis.sh

@@ -52,9 +52,6 @@ elif [ -n "${EMSCRIPTEN}" ]; then
     curl -L "${CARGO_WEB_URL}" | gzip -d > ~/.cargo/bin/cargo-web
     chmod +x ~/.cargo/bin/cargo-web
 
-    # Install Node.js
-    nvm install 9
-
     cd "$DIR/test_suite"
     cargo web test --target=asmjs-unknown-emscripten --nodejs
     cargo web test --target=wasm32-unknown-emscripten --nodejs