Add some commentary around allowed failures.

.travis.yml

@@ -1,9 +1,11 @@
 # Available ruby versions: http://rubies.travis-ci.org/
 
 language: ruby
+
 os:
   - linux
   - osx
+
 rvm:
   - "2.0.0"
   - "2.1" # latest 2.1.x
@@ -13,11 +15,14 @@ rvm:
   - "jruby-9.0.5.0"
   - "jruby-9.1.5.0"
   - "jruby-head"
+
 script:
   bundle exec rake test
+
 branches:
   except:
     - "readme-edits"
+
 before_install:
   # bundler installation needed for jruby-head
   # https://github.com/travis-ci/travis-ci/issues/5861
@@ -25,28 +30,38 @@ before_install:
 
 # Travis OS X support is pretty janky. These are some hacks to include tests
 # only on versions that actually work.
-# (last tested: 2016-04)
+# (last tested: 2016-10)
 matrix:
   exclude:
     - os: osx
       rvm: '2.2'
     - os: osx
       rvm: '2.3.1' # No 2.3.x at all
+
   include:
     - os: osx
-      rvm: '2.2.2' # Travis OS X doesn't have 2.2 aliases
+      rvm: '2.2.5' # Travis OS X doesn't have 2.2 aliases
+
   allow_failures:
+
+    # bundle fails on ruby 2.4 due to rdoc dependency on json
+    # https://github.com/flori/json/issues/303
     - rvm: 'ruby-head'
-    - os: osx
-      rvm: 'jruby-9.0.5.0'
+
+    # jruby 9.1.x.x fails tests due to bug below
     - os: osx
       rvm: 'jruby-head'
     - os: linux
       rvm: 'jruby-head'
+
+    # jruby 9.1.x.x fails tests due to jruby bug
+    # https://github.com/jruby/jruby/issues/4217
     - os: osx
       rvm: 'jruby-9.1.5.0'
     - os: linux
       rvm: 'jruby-9.1.5.0'
+
+  # return results as soon as mandatory versions are done
   fast_finish: true
 
 sudo: false