Update signature for alterTable

Since alterTable is semi-public, the method now checks the arugments to
make sure that it still supports the old signature that doesn't include
actualFks

lib/migration.js

@@ -116,6 +116,12 @@ function mixinMigration(MySQL, mysql) {
   };
 
   MySQL.prototype.alterTable = function(model, actualFields, actualIndexes, actualFks, done, checkOnly) {
+    //if this is using an old signature, then grab the correct callback and check boolean
+    if ('function' == typeof actualFks && typeof done !== 'function') {
+      checkOnly = done || false;
+      done = actualFks;
+    }
+
     var self = this;
     var m = this.getModelDefinition(model);
     var propNames = Object.keys(m.properties).filter(function(name) {