Properly support current Laravel versions

[sergejostir]

This PR does the following:

1. It properly implements deferrable service provider, for Laravel 5.8+ (https://laravel.com/docs/5.8/upgrade#deferred-service-providers).

2. It properly specifies dependencies in composer.json. It also does not use greater or equal to sign (>=) anymore, since considering the semver it is not neccessary that all future Laravel versions will simply work.

The result is that this package is now compatibile only to Laravel 5.8 and newer. That shouldn't be a problem since all earlier versions have reached EOL long time ago already, and if we want to keep this package up to date to current standards that's a small price to pay.

I have also updated README to reflect these changes. The only thing left is to update the Github repository description to say "The .env file editor tool for Laravel 5.8+".

[JackieDo]

@sergiz : I agree with what you said. At the same time I need to consult from you the following:

• Firstly, from now on, we should release new versions with 2.x tags, in order to completely separate from the old releases?
• Second, I plan to move the documentation of this package to using Github Page (put on the gh-pages branch) as I did with another of my package here. Should I do that?

[sergejostir]

a) Considering the semver, there were no incompatible API changes introduced, so it should be a minor release. Here is the article that is often linked: https://www.doctrine-project.org/2017/07/25/php-7.1-requirement-and-composer.html
I guess it should only be noted in the README, what version should people install when using <L5.8.

b) Well, I personally prefer a single page documentation using README for simple packages like this. Your cart package seems pretty complex, so it makes sense to make a multi page docs, but for env-editor where we only have a few methods to document, I find it easier to have everything on a single page.
Also it's easier for people, that are searching for packages, if everything is shown up on the first page, so they can quickly see what it can do, without opening new tabs.

[JackieDo]

a) Considering the semver, there were no incompatible API changes introduced, so it should be a minor release. Here is the article that is often linked: https://www.doctrine-project.org/2017/07/25/php-7.1-requirement-and-composer.html
I guess it should only be noted in the README, what version should people install when using <L5.8.

b) Well, I personally prefer a single page documentation using README for simple packages like this. Your cart package seems pretty complex, so it makes sense to make a multi page docs, but for env-editor where we only have a few methods to document, I find it easier to have everything on a single page.
Also it's easier for people, that are searching for packages, if everything is shown up on the first page, so they can quickly see what it can do, without opening new tabs.

OK, so we will come to the following agreement:

• Only new releases with 1.2.x tags
• Keep the documentation in the README file (but need to standardize the content more carefully).

Thank you for your advice.