BUGFIX: some unigrams were being read incorrectly.

By design, FilePiece invalidates any previous StringPiece when a new read happens.  Read1Gram was reading the vocab word, reading the backoff, and then
using the vocab word.  This ordering was incorrect and led to occasional vocab issues.

Stephan Peitz saw this as a duplicate n-gram detected error since words were mapping to OOV.