Try using defined.

Author: octalmage

.travis.yml

@@ -50,7 +50,7 @@ script:
   - npm test
 
 after_success:
-  - if [ $TRAVIS_TAG != "" ]; then npm run prebuild -- -u $GITHUB_TOKEN; fi
+  - if [[ $TRAVIS_TAG != "" ]]; then npm run prebuild -- -u $GITHUB_TOKEN; fi
 
 notifications:
   webhooks:

appveyor.yml

@@ -32,7 +32,7 @@ test_script:
   - npm test
 
 on_success:
-  - IF "%APPVEYOR_REPO_TAG%" NEQ "" npm run prebuild -- -u %GITHUB_TOKEN%
+  - IF defined "%APPVEYOR_REPO_TAG%" npm run prebuild -- -u %GITHUB_TOKEN%
 
 build: off
 version: "{build}"