Revert "Remove piolibdeps cache"

This reverts commit e009f21a72.

.travis.yml

@@ -4,6 +4,9 @@ language: python
 cache:
   directories:
     - "~/.platformio"
+    - "$TRAVIS_BUILD_DIR/tests/build/test1/.piolibdeps"
+    - "$TRAVIS_BUILD_DIR/tests/build/test2/.piolibdeps"
+    - "$TRAVIS_BUILD_DIR/tests/build/test3/.piolibdeps"
 
 matrix:
   fast_finish: true

tests/test1.yaml

@@ -491,7 +491,7 @@ sensor:
       - mqtt.publish_json:
           topic: the/topic
           payload: |-
-            root["key"] = id(the_sensor).value;
+            root["key"] = id(the_sensor).state;
             root["greeting"] = "Hello World";