Travis: Make JRuby produce test coverage info

  - Enable the environment variable JRUBY_OPTS to have JRuby produce code coverage data
  - Travis: pass `travis-lint` by adding the `language` key in the YAML configuration

Author: olleolleolle

.travis.yml

@@ -1,15 +1,17 @@
+language: ruby
 sudo: false
 
-rvm:
-  - ruby-head
-  - 2.4.1
-  - 2.3.4
-  - 2.2
-  - 2.1
-  - 2.0.0
-  - jruby-9.1.12.0
-
 matrix:
+  include:
+    - rvm: ruby-head
+    - rvm: 2.4.1
+    - rvm: 2.3.4
+    - rvm: 2.2
+    - rvm: 2.1
+    - rvm: 2.0.0
+    - rvm: jruby-9.1.12.0
+      env:
+        - JRUBY_OPTS="--debug"
   allow_failures:
     - rvm: ruby-head
   fast_finish: true