Changed init completion: ignore skipped tasks

no issue

- might fix TryGhost/Ghost#10155
- the init completion is important, because as soon as you run `knex-migrator migrate`, knex-migrator would execute all missing migrations
- we should ignore the skipped tasks, because otherwise the following case does not work
  - you run `knex-migrator init`
  - the process get's destroyed
  - db is locked
  - one init script was executed
  - you release the lock
  - knex-migrator tells you to run "init"
  - you run it again
  - one task was skipped and the db migrations table is incomplete, because the init completion did not run

lib/index.js

@@ -115,8 +115,9 @@ KnexMigrator.prototype.init = function init(options) {
                             }
                         })
                         .then(function () {
-                            // CASE: re-run init and e.g. one migration was added to the init folder.
-                            if (skippedTasks.length || skipInitCompletion) {
+                            // CASE: you can disale init completion (that means, knex-migrator does not insert the existing
+                            //       migration scripts
+                            if (skipInitCompletion) {
                                 return Promise.resolve();
                             }
 
@@ -142,6 +143,7 @@ KnexMigrator.prototype.init = function init(options) {
                                     let filesToMigrateTo = utils.readTasks(versionPath) || [];
 
                                     return Promise.each(filesToMigrateTo, function (fileToMigrateTo) {
+                                        // CASE: check if migration exists, do not insert twice
                                         return transacting('migrations')
                                             .where('name', fileToMigrateTo.name)
                                             .then(function (migrationExists) {