fix(travis): use the ssh_known_hosts addon to run ssh-keyscan

Travis provides a wonderful helper addon documented at
https://docs.travis-ci.com/user/ssh-known-hosts/.

The behavior changed moves the ssh-keyscan from the before_install
stage to the addons stage.

.travis.yml

@@ -4,6 +4,8 @@ git:
   submodules: false
 
 addons:
+  ssh_known_hosts:
+  - github.com
   apt:
     sources:
     - ubuntu-toolchain-r-test
@@ -41,7 +43,6 @@ matrix:
 before_install:
 - openssl aes-256-cbc -k "$DECRYPTION_PASSWORD" -in build/resources/ssh/nylas-n1-ci-ssh.openssl.enc -out nylas-n1-ci-ssh -d
 - mv nylas-n1-ci-ssh ~/.ssh/id_rsa
-- ssh-keyscan github.com >> ~/.ssh/known_hosts
 - chmod 400 ~/.ssh/id_rsa
 - git submodule update --init --recursive
 - 2>/dev/null 1>/dev/null openssl aes-256-cbc -K $encrypted_d583b56b822e_key -iv $encrypted_d583b56b822e_iv -in build/resources/nylas/encrypted_certificates/travis/travis-files.tar.enc -out build/resources/nylas/encrypted_certificates/travis/travis-files.tar -d