Update Travis config.

* Bump Ruby versions.
* Include ActionMailer 4.2 Gemfile.
* Exclude Ruby < 2.2.2 from the matrix for current Gemfile.

.travis.yml

@@ -3,22 +3,32 @@ rvm:
   - 1.8.7
   - 1.9.3
   - 2.0.0
-  - 2.1.5
+  - 2.1.9
+  - 2.2.5
 script: bundle exec rake spec
 gemfile:
   - Gemfile
   - gemfiles/Gemfile.actionmailer-3.0.x
   - gemfiles/Gemfile.actionmailer-3.2.x
   - gemfiles/Gemfile.actionmailer-4.0.x
   - gemfiles/Gemfile.actionmailer-4.1.x
+  - gemfiles/Gemfile.actionmailer-4.2.x
 matrix:
   exclude:
     - rvm: 1.8.7
       gemfile: Gemfile
+    - rvm: 1.9.3
+      gemfile: Gemfile
+    - rvm: 2.0.0
+      gemfile: Gemfile
+    - rvm: 2.1.9
+      gemfile: Gemfile
     - rvm: 1.8.7
       gemfile: gemfiles/Gemfile.actionmailer-4.0.x
     - rvm: 1.8.7
       gemfile: gemfiles/Gemfile.actionmailer-4.1.x
+    - rvm: 1.8.7
+      gemfile: gemfiles/Gemfile.actionmailer-4.2.x
 before_install:
   - gem update --system
   - gem install bundler