Lower the default version to 7.3.1

It looks like there's some additional testing in 7.4.x and 7.5.x we'll need to do, specifically:
- pull the latest packaged init scripts, confirm #726 and #727
- confirm double instances still converges and works #695
- take a look at TravisCI ruby build errors we're seeing in PRs currently
- at some point, also investigate chef 15.x support

README.md

@@ -156,7 +156,7 @@ elasticsearch_install 'elasticsearch'
 ```ruby
 elasticsearch_install 'my_es_installation' do
  type 'package' # type of install
- version '7.5.1'
+ version '7.3.1'
  action :install # could be :remove as well
 end
 ```
@@ -177,7 +177,7 @@ end
 ```ruby
 elasticsearch_install 'my_es_installation' do
  type 'tarball' # type of install
- version '7.5.1'
+ version '7.3.1'
  action :install # could be :remove as well
 end
 ```

libraries/resource_install.rb

@@ -11,7 +11,7 @@ class ElasticsearchCookbook::InstallResource < Chef::Resource::LWRPBase
 
  # if this version parameter is not set by the caller, we look at
  # `attributes/default.rb` for a default value to use, or we raise
- attribute(:version, kind_of: String, default: '7.5.1')
+ attribute(:version, kind_of: String, default: '7.3.1')
 
  # we allow a string or symbol for this value
  attribute(:type, kind_of: String, equal_to: %w(package tarball repository), default: 'repository')

test/integration/helpers/serverspec/install_examples.rb

@@ -2,7 +2,7 @@
 
 shared_examples_for 'elasticsearch install' do |args = {}|
  dir = args[:dir] || '/usr/share'
- version = args[:version] || '7.5.1'
+ version = args[:version] || '7.3.1'
 
  expected_user = args[:user] || 'elasticsearch'
  expected_group = args[:group] || expected_user || 'elasticsearch'