refactor(ssh): delay loading of tunnel-ssh and improve load time

This improves the loadtime of db-migrate by 47%. To be exact from ~22ms to ~15ms.

Resolves #387

lib/driver/index.js

@@ -5,8 +5,7 @@ internals.mod.log = require('db-migrate-shared').log;
 internals.mod.type = require('db-migrate-shared').dataType;
 var Shadow = require('./shadow');
 var log = internals.mod.log;
-var tunnel = require('tunnel-ssh'),
-    Promise = require('bluebird'),
+var Promise = require('bluebird'),
     SeederInterface = require('../interface/seederInterface.js'),
     MigratorInterface = require('../interface/migratorInterface.js'),
     resolve = require( 'resolve' );
@@ -119,6 +118,7 @@ exports.connect = function (config, intern, callback) {
   };
 
   if (config.tunnel) {
+    var tunnel = require('tunnel-ssh');
     var tunnelConfig = JSON.parse(JSON.stringify(config.tunnel));
     tunnelConfig.dstHost = config.host;
     tunnelConfig.dstPort = config.port;
@@ -132,7 +132,6 @@ exports.connect = function (config, intern, callback) {
     config.port = tunnelConfig.localPort;
 
     tunnel(tunnelConfig, function (err) {
-
       if (err) {
         callback(err);
         return;

package.json

@@ -56,7 +56,7 @@
     "pkginfo": "~0.3.0",
     "resolve": "^1.1.6",
     "semver": "~5.1.0",
-    "tunnel-ssh": "^2.0.0"
+    "tunnel-ssh": "^4.0.0"
   },
   "devDependencies": {
     "code": "^1.3.0",