Update metadata version comparison rules in client workflow

The client workflow has a set of version comparison rules for how
to update metadata files. The following PR addresses the differences
coming from the fact that when updating not all metadata files should
be treated equally.

Fixes #207 and is related to #114

Signed-off-by: Radoslav Dimitrov <[email protected]>

Author: rdimitrov

tuf-spec.md

@@ -1315,11 +1315,11 @@ it in the next step.
   and report the signature failure.
 
 5. **Check for a rollback attack.** The version number of the trusted
-  root metadata file (version N) MUST be less than or equal to the version
+  root metadata file (version N) MUST be less than the version
   number of the new root metadata file (version N+1). Effectively, this means
   checking that the version number signed in the new root metadata file is
-  indeed N+1.  If the version of the new root metadata file is less than the
-  trusted metadata file, discard it, abort the update cycle, and report the
+  indeed N+1.  If the version of the new root metadata file is less than or equal
+  to the version of the trusted metadata file, discard it, abort the update cycle, and report the
   rollback attack.
 
 6. Note that the expiration of the new (intermediate) root metadata
@@ -1368,9 +1368,9 @@ it in the next step.
 3. **Check for a rollback attack.**
 
   1. The version number of the trusted timestamp metadata file, if
-    any, MUST be less than or equal to the version number of the new timestamp
-    metadata file.  If the new timestamp metadata file is older than the
-    trusted timestamp metadata file, discard it, abort the update cycle, and
+    any, MUST be less than the version number of the new timestamp
+    metadata file.  If the new timestamp metadata file is less than or equal to
+    the trusted timestamp metadata file, discard it, abort the update cycle, and
     report the potential rollback attack.
 
   2. The version number of the snapshot metadata file in the