[build] upgrade docker in travis build to latest (#4478)

* [build] upgrade docker in travis build to latest
* add comment to the before_install job

---------

Co-authored-by: Gheis Mohammadi <Gheis.Mohammadi@gmail.com>

.travis.yml

@@ -12,6 +12,14 @@ env:
   - TEST="bash ./scripts/travis_go_checker.sh"
   - TEST="bash ./scripts/travis_rpc_checker.sh"
   - TEST="bash ./scripts/travis_rosetta_checker.sh"
+
+before_install:
+  # upgrade docker to the latest supported by the OS loaded in the travis image
+  - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+  - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+  - sudo apt-get update
+  - sudo apt-get -y -o Dpkg::Options::="--force-confnew" install docker-ce
+
 install:
   # default working directory with source code is automatically set to
   #   /home/travis/gopath/src/github.com/harmony-one/harmony