Add Carthage wrapper to circumvent issues with Xcode 12

Carthage is currently not working with Xcode 12. As a workaround a wrapper script has been proposed (see https://gist.github.com/skymobilebuilds/61f4a95bd62a3db36b52719aeab7e0d5). Until the issue is fixed and Carthage can built again with Xcode 12, the wrapper must be used instead of the direct `carthage` command.

.travis.yml

@@ -29,7 +29,7 @@ before_script:
       ls -ld Carthage/Build/iOS/*framework
     else
       echo "Bootstrap carthage dependencies."
-      travis_wait 60 carthage bootstrap --platform iOS
+      travis_wait 80 ./wcarthage bootstrap --platform iOS
     fi
   - |
     if [ ! $(git diff --name-only "$TRAVIS_COMMIT_RANGE" | grep -E "gopenpgp_build.sh") ] && [ -d "go/dist/Crypto.framework" ]; then