Migration sql changes (#199)

* safe migration script changes

* lamassu migration script fixes

migrations/015-paired_devices.js

@@ -2,9 +2,9 @@ var db = require('./db')
 
 exports.up = function (next) {
   var sql = [
-    'alter table devices drop authorized',
-    'alter table devices drop unpair',
-    `create table paired_devices (
+    db.dropColumn('devices', 'authorized'),
+    db.dropColumn('devices', 'unpair'),
+    `CREATE TABLE IF NOT EXISTS paired_devices (
       device_id text PRIMARY KEY,
       created timestamptz NOT NULL default now()
     )`