try to fix travis

.travis.yml

@@ -45,6 +45,8 @@ matrix:
 
     - name: platformio
       script: platformio run -e VARIANT_ADC -e VARIANT_USART3 -e VARIANT_HOVERCAR -e VARIANT_TRANSPOTTER -e VARIANT_NUNCHUCK -e VARIANT_PPM
+      env: 
+        - default_envs=VARIANT_ADC
       language: python
       python:
         - "2.7"