Build 'core' project in CircleCI

Part of #66.

Author: krzema12

.circleci/config.yml

@@ -1,5 +1,13 @@
 version: 2
 jobs:
+  core:
+    docker:
+      - image: circleci/openjdk:8-jdk-node-browsers
+    working_directory: ~/repo
+    steps:
+      - checkout
+      - run: ./gradlew --version
+      - run: ./gradlew :core:build
   android:
     docker:
       - image: circleci/android:api-29
@@ -65,6 +73,7 @@ workflows:
   version: 2
   Tests:
     jobs:
+      - core
       - android
       - cli
       - web