Travis: Use default script step; latest versions in matrix (puma#1414)

* Travis: Use default script step

  - move TESTOPTS=-v to a global env instead

* Travis: ruby 2.2.8, 2.3.5, 2.4.2

.travis.yml

@@ -7,14 +7,13 @@ before_install:
   # bundler installation needed for jruby-head
   # https://github.com/travis-ci/travis-ci/issues/5861
   - gem install bundler
-script: "TESTOPTS=-v bundle exec rake"
 branches:
   only:
     - "master"
 rvm:
-  - 2.2.7
-  - 2.3.4
-  - 2.4.1
+  - 2.2.8
+  - 2.3.5
+  - 2.4.2
   - ruby-head
   - jruby-9.1.13.0
   - jruby-head
@@ -28,3 +27,6 @@ matrix:
     - rvm: jruby-head
     - rvm: rbx-3
     - rvm: ruby-head
+env:
+  global:
+    - TESTOPTS="-v"