Ensure the latest rubygems is installed

Bundler 1.16.1 requires rubygems >= 2.7.4
See travis-ci/travis-ci#8978 (comment)

.travis.yml

@@ -10,6 +10,11 @@ rvm:
   - 2.4.2
   - 2.5.1
 
+before_install:
+  # Ensure we have rubygems >= 2.4.7
+  # https://github.com/travis-ci/travis-ci/issues/8978#issuecomment-354036443
+  - gem update --system
+
 env:
   - 'RAILS_VERSION="~> 4.2"'
   - 'RAILS_VERSION="~> 5.0.0"'
@@ -34,3 +39,11 @@ matrix:
         - 'AF_VERSION="~> 8.0"'
         - 'RAILS_VERSION="~> 4.2"'
       rvm: 2.3.5
+  exclude:
+    - env: # Already explicitly in the include
+      - 'RAILS_VERSION="~> 4.2"'
+      rvm: 2.3.5
+
+
+
+