And by 1.8, I mean MRI 1.8

Gemfile

@@ -28,7 +28,7 @@ platforms :ruby do
 end
 
 # Cross-templating language block fix for Ruby 1.8
-platforms :ruby_18 do
+platforms :mri_18 do
   gem "ruby18_source_location"
 end
 

middleman-core/lib/middleman-core/templates/empty/Gemfile.tt

@@ -3,6 +3,6 @@ source 'https://rubygems.org'
 gem "middleman", "~><%= Middleman::VERSION %>"
 
 # Cross-templating language block fix for Ruby 1.8
-platforms :ruby_18 do
+platforms :mri_18 do
   gem "ruby18_source_location"
 end

middleman-core/lib/middleman-core/templates/shared/Gemfile.tt

@@ -11,6 +11,6 @@ gem "middleman-livereload", "~> 3.1.0"
 # gem "wdm", "~> 0.1.0") # Windows
 
 # Cross-templating language block fix for Ruby 1.8
-platforms :ruby_18 do
+platforms :mri_18 do
   gem "ruby18_source_location"
 end

middleman-core/lib/middleman-more/core_extensions/default_helpers.rb

@@ -1,5 +1,6 @@
 # Required to hack around Padrino blocks within different template types.
-if RUBY_VERSION < "1.9.0"
+require 'rbconfig'
+if RUBY_VERSION =~ /1.8/ && RbConfig::CONFIG['ruby_install_name'] == 'ruby'
   begin
     require 'ruby18_source_location'
   rescue LoadError