remove dependency on Http::Parser#request_path

The upstream releases of the c http_parser no longer expose the request_path method, causing Websocket#dispatch to crash out. 

`lib/guard/livereload/websocket.rb:15:in 'dispatch': undefined method 'request_path' for #<HTTP::Parser:0x007ff21378e670> (NoMethodError)`

Also see: https://github.com/tmm1/http_parser.rb/issues/8

I'm not sure exactly what combination of versions of gems pulls in a recent enough version to cause this to be a problem, as I have a lot of complicated dependencies around em-http-request etc.

Anyway, we don't need to depend on request_path, its trivial to use URI.parse().path to remove this dependency, and make guard-livereload a bit more future proof.