Upgrade to less 2.5 and be less restrictive about the version

I updated the `less` dependency to version 2.5 and at the same time restricted it only to the major version instead of the minor version, because a new minor release should be backwards compatible and it will allow people to upgrade to a newer less version without having to release a new version of this gem.

I am using this version in my project as a git dependency and everything still works as well as all test cases are succeeding.

Mentioning #83: If you really need this urgently, put

```ruby
gem "less-rails", git: "git@github.com:CQQL/less-rails.git", branch: "less-2.5"
```

in your Gemfile.