Drop RVM invoication for xcode 6.4

Author: ignisf

.travis.yml

@@ -12,11 +12,9 @@ matrix:
     - rvm: 2.4.0
       os: osx
       osx_image: xcode7.3
-    - rvm: 2.4.0
-      os: osx
+    - os: osx
       osx_image: xcode6.4
-    - rvm: 2.4.0
-      os: osx
+    - os: osx
       osx_image: xcode9.1
     - rvm: system
       os: osx
@@ -36,6 +34,8 @@ matrix:
   allow_failures:
     - rvm: 1.9.3
     - rvm: rbx
+    - os: osx
+      osx_image: xcode6.4
   fast_finish: true
 dist: trusty
 addons: