Purging occurs for prefixed sources when a 2nd non-prefix source shares basedirs

[diginc]

RE: #53
This seems to bug purging if multiple sources share the same basedir directory and one has disabled prefixing, seems the non-prefixed source remains and the prefixed one gets deleted:

:cachedir: '/var/cache/r10k'
:sources:
  :hieradata: 
    remote: 'git@blabla1.git'
    basedir: '/etc/puppet/environments'
    prefix: true
  :modules: 
    remote: 'git@blabla2.git'
    basedir: '/etc/puppet/environments'
    prefix: false

:purgedirs:
  - '/etc/puppet/environments'

# Results in:
[R10K::Task::Deployment::PurgeEnvironments - INFO] Purging stale environments from /etc/puppet/environments
[R10K::Task::Deployment::PurgeEnvironments - DEBUG] Stale modules in /etc/puppet/environments: hieradata_dev, hieradata_prod, hieradata_master
[R10K::Task::Deployment::PurgeEnvironments - DEBUG] No stale environments in /etc/puppet/environments
# Leaving:
$ ls /etc/puppet/environments/
dev  master  prod

I only bring this up because I had trouble getting a modulepath with a prefix working like:
[master] modulepath = /etc/puppet/environments/modules_$environment/modules

(running latest master version through bundle)

~/r10k/lib/r10k$ git log | head -1
commit 68b8f0ecbce44f4c99c740b8765e5e48306249b4
~/r10k/lib/r10k$ git branch
* master