Run tests on Travis-CI

Rely on Travis for basic test stage. No manual xcodebuild script.

Had to inject a manual "pod repo update" though.

https://github.com/travis-ci/travis-ci/issues/6473#issuecomment-247777762

Extend build time to 20 minutes (OpenSSL takes a lot).

.travis.yml

@@ -0,0 +1,12 @@
+language: objective-c
+osx_image: xcode10.2
+xcode_workspace: TunnelKit.xcworkspace
+xcode_scheme: TunnelKit-iOS
+xcode_destination: platform=iOS Simulator,OS=12.2,name=iPhone 8 Plus
+cache:
+    - bundler
+    - cocoapods
+install:
+    - bundle install --jobs=3 --deployment --path=${BUNDLE_PATH:-vendor/bundle}
+    - bundle exec pod repo update
+    - travis_wait 20 bundle exec pod install