.travis.yml in stellar-base-0.19.0 vs .travis.yml in stellar-base-0.20.0

- old
+ new

@@ -1,13 +1,16 @@
+dist: xenial
 language: ruby
 rvm:
-- 2.5.1
-- 2.6.2
+- 2.5
+- 2.6
+cache: bundler
+addons:
+  apt:
+    packages:
+    - libsodium-dev
 before_install:
-  - sudo add-apt-repository -y ppa:chris-lea/libsodium
-  - sudo apt-get -y update
-  - sudo apt-get install -y libsodium-dev
-  - gem update bundler
+  - gem update --system && gem install --no-document bundler
 script: bundle exec rake travis
 notifications:
   slack:
     secure: V/6a8KFe067uukrbCJA2R1HPO4xy0YSQ1pwmHVRi5StX+yl+lYsWWJdjdBdT0j3iJBYyPRqU4bQYck+OloxtELnrHCX+OkodxcxW8W/ACc914iIf0FyY9pnusK7ck2awmt4Iuf94YPgi0XTm1aCcm+f0yU7wiIVFpftoXSk1EDY=