Fix references to the non-existent variable 'opts' in update_manifest().

This argument was removed in 15ae6838e863750f945103379fdecb52ae26ff1b.
Sort MANIFEST.json.

This avoids spurious changes when updating it.

Trailing whitespace is also removed.
Merge pull request #1133 from Ms2ger/manifest

Update manifest.py; r=jgraham