Bump minimum required Ruby version from 2.3+ to 2.4+

.travis.yml

@@ -1,6 +1,5 @@
 language: ruby
 rvm:
-  - 2.3.0
   - 2.4.0
   - 2.5.0
   - 2.6.0

beef

@@ -12,11 +12,11 @@
 $VERBOSE = nil
 
 #
-# @note Version check to ensure BeEF is running Ruby 2.3+
+# @note Version check to ensure BeEF is running Ruby 2.4+
 #
-if RUBY_VERSION < '2.3'
+if RUBY_VERSION < '2.4'
   puts
-  puts "Ruby version #{RUBY_VERSION} is no longer supported. Please upgrade to Ruby version 2.3 or later."
+  puts "Ruby version #{RUBY_VERSION} is no longer supported. Please upgrade to Ruby version 2.4 or later."
   puts
   exit 1
 end