[Tests] Do not install nodejs, npm deps for SHELLCHECK

.travis.yml

@@ -22,7 +22,7 @@ before_install:
   - curl --version
   - wget --version
   - if [ -n "${SHELLCHECK-}" ]; then cabal update && cabal install transformers-0.4.3.0 ShellCheck && shellcheck --version ; fi
-  - nvm install node && npm install && npm ls urchin doctoc
+  - if [ -z "${SHELLCHECK-}" ]; then nvm install node && npm install && npm ls urchin doctoc; fi
 install:
   - '[ -z "$WITHOUT_CURL" ] || sudo apt-get remove curl -y'
 script: