re-generate Gemfile.lock

I had a dependancy problem when adding alchemy-demo_kit to Gemfile.

```
  Bundler could not find compatible versions for gem "alchemy_cms":
    In Gemfile:
      alchemy-demo_kit (>= 0) ruby depends on
        alchemy_cms (~> 2.5.0.b1) ruby

      alchemy_cms (2.8.1)
```

Problem fixed after regenrating Gemfile.lock