diff --git a/spec/mspec/.travis.yml b/spec/mspec/.travis.yml
index 73f141d2ae..d942de3d21 100644
--- a/spec/mspec/.travis.yml
+++ b/spec/mspec/.travis.yml
@@ -7,10 +7,17 @@ script:
   - bundle exec rspec
 matrix:
   include:
-  - rvm: 2.2.8
-  - rvm: 2.3.5
-  - rvm: 2.4.2
+  - rvm: 2.2.9
+  - rvm: 2.3.6
+  - rvm: 2.4.3
+  - rvm: 2.5.0
+    before_install:
+      # https://github.com/travis-ci/travis-ci/issues/8978
+      - gem update --system
   - rvm: ruby-head
+    before_install:
+      # https://github.com/travis-ci/travis-ci/issues/8978
+      - gem update --system
   - jdk: oraclejdk8
     install:
       - curl -L https://github.com/graalvm/truffleruby/releases/download/vm-enterprise-0.28/truffleruby-testing-0.28.tar.gz | tar xz
diff --git a/spec/mspec/lib/mspec/guards/platform.rb b/spec/mspec/lib/mspec/guards/platform.rb
index 96176b8753..9543b1dd05 100644
--- a/spec/mspec/lib/mspec/guards/platform.rb
+++ b/spec/mspec/lib/mspec/guards/platform.rb
@@ -40,8 +40,21 @@ class PlatformGuard < SpecGuard
     os?(:windows)
   end
 
+  WORD_SIZE = 1.size * 8
+
+  POINTER_SIZE = begin
+    require 'rbconfig/sizeof'
+    RbConfig::SIZEOF["void*"] * 8
+  rescue LoadError
+    WORD_SIZE
+  end
+
   def self.wordsize?(size)
-    size == 8 * 1.size
+    size == WORD_SIZE
+  end
+
+  def self.pointer_size?(size)
+    size == POINTER_SIZE
   end
 
   def initialize(*args)
@@ -61,6 +74,8 @@ class PlatformGuard < SpecGuard
         match &&= PlatformGuard.os?(*value)
       when :wordsize
         match &&= PlatformGuard.wordsize? value
+      when :pointer_size
+        match &&= PlatformGuard.pointer_size? value
       end
     end
     match