(maint) Add Ruby 2.4 to Travis

 - Note that AppVeyor doesn't yet have Ruby 2.4, likely because the
   folks who maintain RubyInstaller haven't yet released a Windows
   installer for Ruby 2.4

 - This PR also updates Travis to run `gem update --system` because
   Rubygems 2.6.9 is required to install the Rainbow gem on Ruby 2.4.
   There may be a better solution to this problem - this is a quick
   spike to detect any spec failures.

   ku1ik/rainbow#44

.travis.yml

@@ -2,12 +2,14 @@ language: ruby
 sudo: false
 before_install:
   - gem install bundler -v 1.10
+  - gem update --system
 bundler_args: --without development
 script:
   - "bundle exec rake $CHECK"
 notifications:
   email: false
 rvm:
+  - 2.4.0
   - 2.3.1
   - 2.2.4
   - 2.1.9
@@ -21,6 +23,8 @@ env:
 
 matrix:
   exclude:
+    - rvm: 2.4.0
+      env: "CHECK=rubocop"
     - rvm: 2.3.1
       env: "CHECK=rubocop"
     - rvm: 2.2.4
@@ -29,6 +33,8 @@ matrix:
       env: "CHECK=rubocop"
     - rvm: 1.9.3
       env: "CHECK=rubocop"
+    - rvm: 2.4.0
+      env: "CHECK=commits"
     - rvm: 2.3.1
       env: "CHECK=commits"
     - rvm: 2.2.4