Remove last-modified from headers as it causes a failure when updating the daily badge (#4522)

tools/travis.ps1

@@ -50,7 +50,6 @@ function Set-DailyBuildBadge
     $headers = @{
         "x-ms-date"      = $now
         "cache-control"  = "no-cache"
-        "last-modified"  = $now
         "x-ms-blob-type" = "BlockBlob"
         "x-ms-version"   = "$headerDate"
     }
@@ -220,4 +219,4 @@ else
     }
 
     Start-PSxUnit
-}
+}