travis: explicitely specify build directory

Signed-off-by: julian <[email protected]>

.travis.yml

@@ -10,13 +10,15 @@ install:
 - pip install -U platformio
 
 script:
-- pio ci --project-conf=platformio.ini src/
+- export BUILD_DIR=$(mktemp -d)
+- pio ci --build-dir=$BUILD_DIR --keep-build-dir --project-conf=platformio.ini src/
 
 deploy:
   provider: releases
   api_key:
     secure: cbbgalc9FMreQNVyGbxspaJgSwBw93HUrXQd03i85U3X+FLQtdwnNCbVdhjAi1vdCUHmS1jeWFXe3tTk2RGyiJGFUlxY3No4tgfxk7rXJv439YszIPUunoRlZGqQFJC4ADYUZDcyP7tus5zA1bKku+oc3K2Z2ZX2nF8GT9RSc0zvYryEn6ioluD/HpNzIRF5OUEMyvn9vNreVha6ksmAI9/T9GLMukkG6l3vhMI+OYodBECiBcgG9FTqVdMexWddnKkUe/Gfd1NWWLTzdoQiYvQ4eckAKWjzDGNP7hvbw37pfKob4cQV2/Dm6p0tsQnE9B+c+giZ/KsaN/nu0nH2cm3N+rKfk2eGbnWqQbP7hTp9UrjS2JgQG8462pxbbL27ojqupmd78sSN1GggWBcrw3N6x6v645NncAdU5eliN5kqk4w2jRBrOqCu+AEl94wJncypmIvWup4z/Tjj+EuZg2xvjZ7A7aCA6ymdeFJ1zvP1qTWxCB36xr+iJZTuuENbbPxjEzY3araHifkNaKYhFgpfprXxhkgKVNV/wjjBarLaPUKOdi+f4qoWIXQYjcHDh+9B7TaYE3RskDJinyXDVvmillzG+zsCyhTEZvClVzEsh2fB5RcS7Gtugt67EyJMLqTr4peHYcC6b+t8TjsngwJoVxgmRBxsaHMOs80ZVsI=
-  file: ".pioenvs/usb/firmware.bin"
+  file:
+    - $BUILD_DIR/.pioenvs/usb/firmware.bin
   skip_cleanup: true
   on:
     repo: nightvisi0n/sonoff-s20