Add JRuby and Truffleruby to Travis-CI

They currently fail, but this is in preparation of ffi#660.

.travis.yml

@@ -25,6 +25,8 @@ matrix:
     - rvm: system
     - os: osx
       rvm: ruby-head
+    - rvm: jruby
+    - rvm: truffleruby
   exclude: # ruby 2.4.2 needs build with xcode9 or later on osx
     - os: osx
       rvm: 2.4.2
@@ -39,5 +41,13 @@ matrix:
       rvm: 2.4.2
       env:
       - CC=clang
+    - os: linux
+      rvm: jruby
+    - os: linux
+      rvm: truffleruby
+    - os: osx
+      rvm: jruby
+    - os: osx
+      rvm: truffleruby
 after_failure:
   - "find build -name mkmf.log | xargs cat"