remove 1.8.7 and use 2.3.5 for CI

.travis.yml

@@ -1,13 +1,12 @@
 language: ruby
 rvm:
-  - 2.3.0
+  - 2.3.5
   - 2.2.3
   - 2.1.7
   - 2.0.0
   - 1.9.3
-  - 1.8.7
 matrix:
   fast_finish: true
   allow_failures:
-    - rvm: 2.3.0
+    - rvm: 2.3.5
 script: "./script/cibuild"