Keep composer updated on boxes

[becw]

Add a trigger to update the installed version of composer on vagrant up (#68).

This updates Composer itself, not the project dependencies, which should be generally good because Composer gets regular performance improvements and bug fixes.

In the infrequent cases where there's a breaking change in Composer (e.g. a schema change), it forces you to keep your project's composer.json up to date -- which is correct, because the general assumption is that any particular environment is running the latest composer.

[becw]

So the feedback on this that we discussed in slack was:

• this is a good idea
• but folks who weighed in would prefer that the Vagrantfile didn't run composer install -- it should just bring the machine up, and shouldn't touch your project

I'll make a new PR to reflect that.