Match release names and numbers in php_spec.rb

Author: ekohl

spec/classes/mod/php_spec.rb

@@ -100,7 +100,7 @@
               }
             end
           when '16.04'
-            context 'on stretch' do
+            context 'on xenial' do
               let :params do
                 { content: 'somecontent' }
               end
@@ -113,7 +113,7 @@
             end
 
           when '18.04'
-            context 'on stretch' do
+            context 'on bionic' do
               let :params do
                 { content: 'somecontent' }
               end