Added alias to rake db:test:prepare

Quite often, db:test:prepare is run after db:migrate.
Added for convenience.

plugins/rails/rails.plugin.zsh

@@ -5,6 +5,7 @@ alias sp='ruby script/plugin'
 alias sr='ruby script/runner'
 alias ssp='ruby script/spec'
 alias rdbm='rake db:migrate'
+alias rdbtp='rake db:test:prepare'
 alias sc='ruby script/console'
 alias sd='ruby script/server --debugger'
 alias devlog='tail -f log/development.log'