Configure CI to build in master branch only

Author: jioo

.circleci/config.yml

@@ -1,6 +1,9 @@
 version: 2
 jobs:
   build:
+    branches:
+      only:
+        - master
     docker:
       - image: microsoft/dotnet:2.2-sdk
     steps:
@@ -16,4 +19,4 @@ jobs:
           command: dotnet cake build.cake --task="Build project"
       - run:
           name: Test
-          command: dotnet cake build.cake --task="Run Test"
+          command: dotnet cake build.cake --task="Run Test"