Fix CI caching syntax

.travis.yml

@@ -4,12 +4,11 @@ xcode_workspace: TunnelKit.xcworkspace
 xcode_scheme: TunnelKit-iOS
 xcode_destination: platform=iOS Simulator,OS=14.2,name=iPhone 11 Pro Max
 cache:
-    - bundler
-    - cocoapods
+  bundler: true
+  cocoapods: true
 branches:
   only:
     - master
-        - stable
 install:
   - bundle install --jobs=3 --deployment --path=${BUNDLE_PATH:-vendor/bundle}
   - bundle exec pod repo update