Remove composer.lock from .gitignore as a part of the build.

Not sure what you think about this, Taylor, but I know people have asked about removing `composer.lock` from `.gitignore` often (e.g. https://github.com/laravel/laravel/pull/3019). I know we keep it in because it's good for the framework, but I figured this build step could help start projects out on the right foot.

Only problem is it now creates an inconsistency--projects installed via the installer get it removed, those installed with composer create-project don't.

Just wanted to suggest it, though, and see what you think.

I tried this with `sed` but had some trouble with it. Theoretically it should've (I think) been:

```
sed -i '/composer\.phar/d' ./.gitignore
```