Update Travis config

[calvellido]

Travis CI is giving some warnings regarding the current config:

• ⚠ root: deprecated key sudo (The key sudo has no effect anymore.)
• ⚠ jobs.include.deploy: deprecated key skip_cleanup (not supported in dpl v2, use cleanup)
• ℹ️ jobs.include.deploy: key local-dir is not underscored, using local_dir
• ℹ️ jobs.include.deploy: key keep-history is not underscored, using keep_history
• ℹ️ root: missing os, using the default osx
• ℹ️ jobs.include.deploy: key skip-cleanup is not underscored, using skip_cleanup
• ℹ️ language: value swift is an alias for objective-c, using objective-c
• ℹ️ jobs.include.deploy: key github-token is an alias for token, using token
• ℹ️ jobs.include.deploy: key target-branch is not underscored, using target_branch

This PR makes suggested changes on the Travis config.

[calvellido]

As this is not urgent, and the CI/CD flow is working right, we can wait to merge this next week :+1: