Use frozen string literal feature when running tests

.travis.yml

 language: ruby
-rvm:
-  - 1.9.3
-  - 2.0.0
-  - 2.1
-  - 2.2
-  - 2.3
-  - 2.4
-  - 2.5
-  - ruby-head
-  - jruby-19mode
-  - rbx-3
+jobs:
+  include:
+    - rvm: 1.9.3
+    - rvm: 2.0.0
+    - rvm: 2.1
+    - rvm: 2.2
+    - rvm: 2.3
+    - rvm: 2.4
+      env: RUBYOPT="--enable-frozen-string-literal"
+    - rvm: 2.5
+      env: RUBYOPT="--enable-frozen-string-literal"
+    - rvm: ruby-head
+      env: RUBYOPT="--enable-frozen-string-literal"
 before_install:
   # 1. The pre-installed Bundler version on Travis is very old; causes 1.9.3 build issues
   # 2. Bundler 2.0 is not supported by the whole matrix
   - gem install bundler -v'< 2'
-matrix:
-  allow_failures:
-    - rvm: ruby-head
-    - rvm: rbx-3
+
+script:
+  -  bundle exec rake

lib/mimemagic.rb

+# frozen_string_literal: true
+
 require 'mimemagic/tables'
 require 'mimemagic/version'
 
@@ -113,7 +115,7 @@ class MimeMagic
 
     io.binmode if io.respond_to?(:binmode)
     io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
-    buffer = "".force_encoding(Encoding::BINARY)
+    buffer = "".encode(Encoding::BINARY)
 
     MAGIC.send(method) { |type, matches| magic_match_io(io, matches, buffer) }
   end