Bump VirtualBox VM memory allocation to 1024 from 512

1GB seems to be a nicer answer than 512MB by default. A Customfile
can be used to reduce or raise the memory footprint to another
level.

Fixes #427
Merge pull request #439 from Varying-Vagrant-Vagrants/update/memory

[PR] Bump VirtualBox VM memory allocation to 1024 from 512