Fix broken Travis builds

Recent deprecations in RubyGems and version constraints in Bundler 2.0.0
require us to be more explicit in our build steps in order for builds to
pass. Skip builds on certain versions and specify an explicit constraint
when installing Bundler.

.travis.yml

@@ -20,8 +20,9 @@ cache: bundler
 
 before_install:
   - "echo 'gem: --no-document' > ~/.gemrc"
-  - gem update --system
-  - gem install bundler  # Necessary to fix 1.9.3
+  # RubyGems update is supported for Ruby 2.3 and later
+  - if $(ruby -e "puts Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.3')"); then gem update --system; fi
+  - gem install bundler --version '~> 1.17'
 
 env:
   global: