Update sysutils/capistrano to latest version, 2.6.0

Upstream changes:

## 2.6.0 / May 3 2011

A rather large release, feature-version bump because of the new
multiple-gateways feature as implemented by Ryan Duryea (way to go!)

Please also note from this release that if you use Git submodules, the
Git-version requirement for the new implementation is now >= 1.5.6, from
previously un-documented. (1.5.6 is new-enough that I think this is
acceptable)

* Upgrade Net::SSH-gateway dependency to 1.1 (fixes a thread-deadlocking bug on
MRI 1.9)
* Respect "dry-run" on transfer methods (Florian Frank)
* Add support for multiple gateways: (Ryan Duryea)
        set :gateway, {
          'gate1.example.com' => 'server1.example.com',
          [ 'gate2.example.com', 'gate3.example.com' ] =>
[ 'server5.example.com', 'server6.example.com' ]
        }
* Properly support nested Git submodules, moves Git requirement to >= 1.5.6 [if
you rely upon submodules] (Ken Miller)
* Fetch tags into the remote cache, allows deploying a tag when using Git, with
the remote_cache strategy (Florian Frank)
* Various fixes to path handling bugs in the copt strategy. (Philippe Rath�)

(gls)