Remove middleman-core dependency in Gemfile

```middleman-core``` declared dependency at ```middleman-blog.gemspec```, no need in ```Gemfile```

Keeping ```gem 'middleman-core'```, ```bundle``` will raise
```
Bundler could not find compatible versions for gem "middleman-core":
  In Gemfile:
    middleman-blog (>= 0) ruby depends on
      middleman-core (~> 3.2) ruby

    middleman-core (4.0.0.pre.0)
```
cause ```middleman-core``` has bumped to 4.x in its master branch