Migration sql changes (#199)

* safe migration script changes

* lamassu migration script fixes

migrations/025-create_trades.js

@@ -2,8 +2,8 @@ var db = require('./db')
 
 exports.up = function (next) {
   var sql = [
-    "create type trade_type as enum ('buy', 'sell')",
-    `create table trades (
+    db.defineEnum('trade_type', "'buy', 'sell'"),
+    `create table if not exists trades (
       id serial PRIMARY KEY,
       type trade_type not null,
       crypto_code text not null,