Add JRuby and Truffleruby to Travis-CI

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

.travis.yml

@@ -5,7 +5,7 @@ language: ruby
 before_install:
   - gem install bundler
 script:
-  - bundle exec rake compile || bundle exec rake compile
+  - if [ -z "$NO_RAKE_COMPILE" ]; then bundle exec rake compile || bundle exec rake compile; fi
   - bundle exec rake test
 os:
   - linux
@@ -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 +41,17 @@ matrix:
       rvm: 2.4.2
       env:
       - CC=clang
+    - os: linux
+      rvm: jruby
+      env:
+      - NO_RAKE_COMPILE=1
+    - os: osx
+      rvm: jruby
+      env:
+      - NO_RAKE_COMPILE=1
+    - os: linux
+      rvm: truffleruby
+    - os: osx
+      rvm: truffleruby
 after_failure:
   - "find build -name mkmf.log | xargs cat"