Revert "Bump version to 1.5.1.0 (#164)" (#165)

This reverts commit c767271.

Author: HashidaTKS

FlexConfirmMail.manifest.xml

@@ -7,7 +7,7 @@
   xsi:type="MailApp">
 
   <Id>20ecd0b2-4157-492b-8d0b-475b729f48eb</Id>
-  <Version>1.6.0.0</Version>
+  <Version>1.5.0.0</Version>
   <ProviderName>ClearCode</ProviderName>
   <DefaultLocale>en-US</DefaultLocale>
   <DisplayName DefaultValue="FlexConfirmMail"/>
@@ -150,4 +150,4 @@
       </Resources>
     </VersionOverrides>
   </VersionOverrides>
-</OfficeApp>
+</OfficeApp>