Fix composer.json

[fritzmg]

The current composer.json for the 1.0.8 tag erroneously contains "version": "1.0.6", which causes Composer to only install version 1.0.7 instead, even though a newer version is available. The version key should be omitted from the composer.json for packages that are version controlled and distributed via either a public or private packagist (see here).

I also streamlined the PHP requirements. ~7.0.6 already excludes 7.0.2 and 7.0.4 and ~7.1 already includes any 7.* version equal or higher than 7.1.0.