Install necessary libraries during travis build

.travis.yml

@@ -24,6 +24,8 @@ script:
   - which arduino
   - source hardware/esp8266com/esp8266/tests/common.sh
   - arduino --board esp8266com:esp8266:generic --save-prefs
+  - arduino --get-pref sketchbook.path
+  - install_libraries
   - build_sketches arduino $PWD/hardware/esp8266com/esp8266
 
 notifications:

tests/common.sh

@@ -20,3 +20,13 @@ function build_sketches()
         fi
     done
 }
+
+function install_libraries()
+{
+    pushd libraries
+
+    # install ArduinoJson library
+    wget https://github.com/bblanchon/ArduinoJson/releases/download/v4.6.1/ArduinoJson-v4.6.1.zip && unzip ArduinoJson-v4.6.1.zip
+
+    popd
+}