Add Dangerfile and make it run on CI

.travis.yml

@@ -9,3 +9,4 @@ install:
 script:
   - swiftlint
   - ./scripts/build.sh
+  - bundle exec danger

Dangerfile

@@ -0,0 +1,17 @@
+## Let's check if there are any changes in the project folder
+has_app_changes = !git.modified_files.grep(/SwiftyStoreKit/).empty?
+
+## Then, we should check if tests are updated
+has_test_changes = !git.modified_files.grep(/SwiftyStoreKitTests/).empty?
+
+# Make it more obvious that a PR is a work in progress and shouldn't be merged yet
+warn("PR is classed as Work in Progress") if github.pr_title.include? "[WIP]"
+
+# Warn when there is a big PR
+warn("Big PR") if git.lines_of_code > 500
+
+## Finally, let's combine them and put extra condition
+## for changed number of lines of code
+if has_app_changes && !has_test_changes && git.lines_of_code > 20
+  fail("Tests were not updated", sticky: false)
+end