Fix Travis mime-types dependency issue

Before this fix some of our Travis builds failed with:

    Gem::InstallError: mime-types-data requires Ruby version >= 2.0.

Idea for the fix was found here:
sgruhier/foundation_rails_helper#116
Although we could not use the same method as I only want to lock
down the version of mime-type where necessary. Also trying to lock
the version to 2.6.2 would break our Rails-3.2 Appraisal path.

Appraisals

@@ -4,14 +4,26 @@ end
 
 appraise 'rails-4.0' do
   gem 'rails', '~> 4.0.0'
+
+  # The last version that doesn't need Ruby 2.0 and works with version 4.0 of
+  # Rails. This addresses a build problem with Travis for version 1.9.3 of Ruby
+  gem 'mime-types', '2.6.2', :platforms => :ruby_19
 end
 
 appraise 'rails-4.1' do
   gem 'rails', '~> 4.1.0'
+
+  # The last version that doesn't need Ruby 2.0 and works with version 4.1 of
+  # Rails. This addresses a build problem with Travis for version 1.9.3 of Ruby
+  gem 'mime-types', '2.6.2', :platforms => :ruby_19
 end
 
 appraise 'rails-4.2' do
   gem 'rails', '~> 4.2.0'
+
+  # The last version that doesn't need Ruby 2.0 and works with version 4.2 of
+  # Rails. This addresses a build problem with Travis for version 1.9.3 of Ruby
+  gem 'mime-types', '2.6.2', :platforms => :ruby_19
 end
 
 appraise 'mongoid-3.0' do

gemfiles/rails_4.0.gemfile

@@ -3,5 +3,6 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 4.0.0"
+gem "mime-types", "2.6.2", :platforms => :ruby_19
 
 gemspec :path => "../"

gemfiles/rails_4.1.gemfile

@@ -3,5 +3,6 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 4.1.0"
+gem "mime-types", "2.6.2", :platforms => :ruby_19
 
 gemspec :path => "../"

gemfiles/rails_4.2.gemfile

@@ -3,5 +3,6 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 4.2.0"
+gem "mime-types", "2.6.2", :platforms => :ruby_19
 
 gemspec :path => "../"