Consider defaulting version to :using => :path

[dblock]

The previous release behaved with

`version 'v1', :using => :path`

Anyone upgrading Grape is going to have to add :using => :path in their versions, maybe worth defaulting to this.

[amrnt]

I think its here now: 0529e92#L1R80

[dblock]

Not on Frontier. It will be a clearly breaking change, https://github.com/intridea/grape/blob/frontier/lib/grape/api.rb.

[mbleigh]

I'm leaving things as-is on frontier because I want this to be the default going forward and so the next major release will be this way. I reverted the default on master.