Allow the location of the bundle to be configured

- Sets vendor/bundle as default
- Uses ~/bundle when generating general.yml
Remove Vagrant reliance on install-script branch

```irc
2014-03-07T11:28:33+0000] <gareth> Why is the branch being replaced 
here? 
https://github.com/mysociety/alaveteli/blob/rails-3-develop/config/Vagra
ntfile#L18
[2014-03-07T11:29:55+0000] <gareth> It can't checkout install-script 
branch as it doesn't exist
[2014-03-07T11:32:10+0000] <gareth> not sure how/why it could do this 
when it fetched a new copy of the repo rather than using the host 
checkout
[2014-03-07T11:35:42+0000] <louise> gareth that may be my fault - I 
deleted a bunch of what I thought were old branches earlier this week - 
perhaps install-script was one of them - let me look
[2014-03-07T11:36:58+0000] <louise> nope, doesn't look like it was - 
mark, should the install-script branch exist? 
[2014-03-07T11:38:18+0000] <mark> what the install script does is to 
use the first of these that can be checked out: origin/install-script 
0.15 origin/master
[2014-03-07T11:38:46+0000] <mark> but i must have created that 
Vagrantfile while the install-script branch existed and worked, but the 
master branch wouldn't
[2014-03-07T11:38:56+0000] <mark> i think now it'd be safe to take that 
line out of the vagrant file
```