improved readme

Just some minor cosmetic improvements.

- link to the Composer homepage (much more useful for beginners)
- do not require changing the version (that has been horribly outdated) on every new minor release
- warn about the evilness of dev-*
- moved Composer autoloading note (it was misleading in the GitHub section, as cloning the repository and then including the project's Composer autoloader will *not* make it load raven-php)