widen specifier on travis versions

Author: Floppy

.travis.yml

@@ -2,7 +2,7 @@ language: ruby
 cache: bundler
 sudo: false
 rvm:
-- 2.4.10
+- 2.4
 services: mongodb
 env:
   global:
@@ -11,4 +11,4 @@ env:
     - PUSHER_SECRET=abc123
     - PUSHER_CLUSTER=eu
 before_install:
-  - gem install bundler -v "1.17.3"
+  - gem install bundler -v "1.17.3"