Fix version prefix in gitian build

The version prefix 'v' should just be set constantly.
Reflect this change in the README as well.

This should allow building commits as well, if a commit
is passed in instead of a tag.

contrib/gitian/gitian-build.py

@@ -165,7 +165,7 @@ def main():
     # Add leading 'v' for tags
     if args.commit and args.pull:
         raise Exception('Cannot have both commit and pull')
-    args.commit = ('' if args.commit else) + args.version
+    args.commit = args.commit if args.commit else args.version
 
     if args.setup:
         setup()