Dependency on abandoned package

[piotrekkr]

When installing deployer using composer I get:

Package guzzle/parser is abandoned, you should avoid using it. Use guzzle/guzzle instead.

[kenjis]

See #259 (comment)

[antonmedv]

Ref reactphp/http-client#23

[johnny-bit]

So... Deployer depends on elfet/pure which in turns depends on react/react, which in turn depends on react/http-client which in turn depends on guzzle/parser... Fun.