Fix TypeError when revision_count is stored as TEXT

The previous schema migration added revision_count as TEXT. SQLite stores
the default as TEXT, but the app does integer arithmetic on it, causing:

  TypeError: unsupported operand type(s) for -: 'int' and 'str'

- init_db() now adds revision_count as INTEGER.
- app.py casts revision_count to int in play() and revise() and when
  incrementing in revise(), so existing TEXT values also work.

Bump version 0.4.2 -> 0.4.3.

VERSION

@@ -1 +1 @@
-0.4.2
+0.4.3