adding SwiftPM to travis CI

Author: swhitty

.travis.yml

@@ -9,3 +9,6 @@ jobs:
       name: macOS
       script: xcodebuild clean test -project SwiftDraw.xcodeproj -scheme SwiftDraw-macOS -sdk macosx
       after_success: bash <(curl -s https://codecov.io/bash) -J 'SwiftDraw'
+    - stage: test
+      name: Swift PM
+      script: swift package clean; swift test