More useful Composer installation instructions

First, thanks for not advising people to add stuff manually to their `composer.json` and run `composer update` - that's just a completely wrong way to *install* packages. But you can do even better - the more appropriate version specifier (assuming semver) is `~2.3`, and not `~2.3.0` - the latter expands to `>=2.3.0, <2.4`, which is not what we want - we want to keep getting new features until there's a BC break (which would be version `3.0.0`).

So the correct way to install packages that follow semver (as yours does, good job!) is to run `composer require vendor/package` - *without* a version specifier - this will install the latest stable release and add the correct version specifier in `composer.json` (e.g. `~X.Y`). Additionally, this would free you from having to update installation instructions each time you release a new version.

For more info see here: http://blog.doh.ms/2014/10/13/installing-composer-packages/
Merge pull request #59 from levacic/patch-1

Remove version specifier from 'composer require' since it's automatically added