Fix build factory

Required column should be set
Refactor job class
Load Gemfile.local if exists
Git plugin was moved to github.com/r7kamura/magi-git

If you want to try it, add magi-git gem into Gemfile.local

```ruby
gem "magi-git"
```
Update README about plugins
SimpleCov plugin was moved to github.com/r7kamura/magi-simple_cov
Update README
v0.1.3