On release, build old Xcodes with Travis

.travis.yml

@@ -0,0 +1,35 @@
+os: osx
+language: swift
+osx_image: xcode9.4
+
+# if: tag =~ /^[0-9]+\.[0-9]+\.[0-9]+/
+
+stages:
+  - swiftpm
+  - carthage
+
+jobs:
+  include:
+    - &swiftpm
+      stage: swiftpm
+      osx_image: xcode9.4
+      script: swift build
+    - <<: *swiftpm
+      osx_image: xcode10.3
+    - <<: *swiftpm
+      osx_image: xcode11.6
+    - <<: *swiftpm
+      osx_image: xcode12.5
+
+    - &carthage
+      stage: carthage
+      osx_image: xcode9.4
+      install: sed -i '' "s/SWIFT_TREAT_WARNINGS_AS_ERRORS = NO;/SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;/" *.xcodeproj/project.pbxproj
+      script: carthage build --no-skip-current
+    - <<: *carthage
+      osx_image: xcode10.3
+    - <<: *carthage
+      osx_image: xcode11.6
+    - <<: *carthage
+      osx_image: xcode12.5
+      script: carthage build --no-skip-current --use-xcframeworks