Fix the warning while building an engine gemspec

After a new plugin is created, if you try to build it with the default generated engine gemspec, the gem(2.2.2) will warn you like this,

```shell
$ gem build engine.gemspec
WARNING:  pessimistic dependency on rails (~> 4.1.0) may be overly strict
  if rails is semantically versioned, use:
    add_runtime_dependency 'rails', '~> 4.1', '>= 4.1.0'
WARNING:  open-ended dependency on sqlite3 (>= 0, development) is not recommended
  if sqlite3 is semantically versioned, use:
    add_development_dependency 'sqlite3', '~> 0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
```
This fix makes the default engine gemspec follow the recommended
semantical version string for dependency to avoid warnings.