Travis CI dependencies caching

[gmariotti]

This issue is for tracking all projects that need to have their .travis.yml updated based on this discussion

In practice it means adding the following lines

branches:
  only:
  - master
  - /^\d+\.\d+$/
----- FROM HERE -----
cache:
  directories:
    - $HOME/.m2
before_cache:
  - rm -rf $HOME/.m2/repository/io/vertx/
----- TO HERE -----
jobs:

Projects:

• [x] vertx-amqp-client
• [x] vertx-kotlin-lang
• [x] vertx-web
• [x] vertx-junit5
• [x] vertx-rabbitmq-client
• [x] vertx-sql-common
• [x] vertx-unit
• [x] vertx-jdbc-client
• [x] vertx-mongo-client
• [x] vertx-service-proxy
• [x] vertx-health-check
• [x] vertx-kafka-client
• [x] vertx-proton
• [x] vertx-reactive-streams
• [x] vertx-redis-client
• [x] vertx-auth
• [x] vertx-grpc
• [x] vertx-consul-client
• [x] vertx-dropwizard-metrics
• [x] vertx-circuit-breaker
• [x] vertx-mqtt
• [x] vertx-micrometer-metrics
• [x] vertx-bridge-common
• [x] vertx-mail-client
• [x] vertx-service-factory
• [x] vertx-stomp
• [x] vertx-tcp-eventbus-bridge
• [x] vertx-camel-bridge
• [x] vertx-rx
• [x] vertx-http-service-factory
• [x] vertx-lang-js
• [x] vertx-lang-groovy
• [x] vertx-lang-ruby
• [x] vertx-config
• [x] vertx-service-discovery
• [x] vertx-maven-service-factory
• [x] vertx-sync
• [x] vertx-sockjs-service-proxy
• [x] vertx-cassandra-client
• [x] vertx-lang-scala
• [x] vertx-hazelcast
• [x] vertx-infinispan
• [x] vertx-amqp-bridge
• [x] vertx-embedded-mongo-db
• [x] vertx-client-services
• [x] vertx-mysql-postgresql-client
• [x] vertx-jca

[slinkydeveloper]

Done