upgrade to 2.4+ rubies

.ruby-version

@@ -1 +1 @@
-2.3.4
+2.6.1

.travis.yml

@@ -4,5 +4,7 @@ cache: bundler
 script:
   - CODECLIMATE_REPO_TOKEN=40ed7f464af09c892b27adc2cd171c36b24992d8d527428285bc55c46fe2f0e0 ruby test/test_all.rb
 rvm:
-  - 2.2.7
-  - 2.3.4
+  - 2.4.5
+  - 2.5.3
+  - 2.6.1
+  

lib/rx-file/util.rb

@@ -7,7 +7,7 @@ module RxFile
     # hence int/bool/string etc are values
     def is_value? o
       return true if [true , false ,  nil].include?(o)
-      return true if [Fixnum, Symbol, String, Class].include?(o.class)
+      return true if [::Integer, Symbol, String, Class].include?(o.class)
       if o.respond_to? :is_value?
         return true if o.is_value?
       end