Update capistrano to 3.8.1.

## `3.8.1` (2017-04-21)

https://github.com/capistrano/capistrano/compare/v3.8.0...v3.8.1

### Breaking changes:

* None

### Fixes:

* [#1867](https://github.com/capistrano/capistrano/pull/1867): Allow `cap -T` to run without Capfile present - [@mattbrictson](https://github.com/mattbrictson)

## `3.8.0` (2017-03-10)

https://github.com/capistrano/capistrano/compare/v3.7.2...v3.8.0

### Potentially breaking changes:

* [#1846](https://github.com/capistrano/capistrano/pull/1846): add_host will add a new host in a case where it used to incorrectly update an existing one (potentially breaking) [(@dbenamy)](https://github.com/dbenamy)

### New features:

* [#1860](https://github.com/capistrano/capistrano/pull/1860): Allow cap to be run within subdir and still work - [@mattbrictson](https://github.com/mattbrictson)

### Fixes:

* [#1835](https://github.com/capistrano/capistrano/pull/1835): Stopped printing parenthesis in ask prompt if no default or nil was passed as argument [(@chamini2)](https://github.com/chamini2)
* [#1840](https://github.com/capistrano/capistrano/pull/1840): Git plugin: shellescape git_wrapper_path [(@olleolleolle)](https://github.com/olleolleolle)
* [#1843](https://github.com/capistrano/capistrano/pull/1843): Properly shell escape git:wrapper steps - [@mattbrictson](https://github.com/mattbrictson)
* [#1846](https://github.com/capistrano/capistrano/pull/1846): Defining a role is now O(hosts) instead of O(hosts^2) [(@dbenamy)](https://github.com/dbenamy)
* Run `svn switch` to work with svn branches if repo_url is changed
* [#1856](https://github.com/capistrano/capistrano/pull/1856): Fix hg repo_tree implementation - [@mattbrictson](https://github.com/mattbrictson)
* [#1857](https://github.com/capistrano/capistrano/pull/1857): Don't emit doctor warning when repo_tree is set - [@mattbrictson](https://github.com/mattbrictson)

### Other changes:

* [capistrano-harrow#4](https://github.com/harrowio/capistrano-harrow/issues/4): Drop dependency on `capistrano-harrow` gem. Gem can still be installed separately [(@leehambley)](https://github.com/leehambley)
* [#1859](https://github.com/capistrano/capistrano/pull/1859): Move git-specific repo_url logic into git plugin - [@mattbrictson](https://github.com/mattbrictson)
* [#1858](https://github.com/capistrano/capistrano/pull/1858): Unset the :scm variable when an SCM plugin is used - [@mattbrictson](https://github.com/mattbrictson)

## `3.7.2` (2017-01-27)

https://github.com/capistrano/capistrano/compare/v3.7.1...v3.7.2

### Potentially breaking changes:

* None

### Other changes:

* Suppress log messages of `git ls-remote` by filtering remote refs (@aeroastro)
* The Git SCM now allows the repo_url to be changed without manually wiping out the mirror on each target host first (@javanthropus)


(taca)