added build dir variable

Author: squix78

.travis.yml

@@ -12,7 +12,7 @@ install:
 - pip install -U platformio
 - platformio lib install 561 562
 script:
-- platformio ci --verbose --keep-build-dir --lib="." --board=nodemcuv2
+- platformio ci --build-dir="./build/" --keep-build-dir --lib="." --board=nodemcuv2
 - ls -altr
 - find . -name firmware.elf