[5.4] Make PhpRedis connection more compatible with Predis (pipeline, ...)

[alibo]

This PR makes PhpRedis connection more compatible with Predis. It allows you to use closure for pipeline or transaction. Also for raw commands, it adds executeRaw, same as Predis library.

1. Pipeline:

$result = Redis::pipeline(function ($pipe) {
   $pipe->set('test:pipeline:1', 1);
   $pipe->get('test:pipeline:1');
   $pipe->set('test:pipeline:2', 2);
   $pipe->get('test:pipeline:2');
});

dump($result);

array:4 [
  0 => true
  1 => "1"
  2 => true
  3 => "2"
]

PhpRedis docs: https://github.com/phpredis/phpredis#multi-exec-discard
Predis docs: https://github.com/nrk/predis#command-pipelines

2. Transaction:

$result = Redis::transaction(function ($pipe) {
   $pipe->set('test:transaction:1', 1);
   $pipe->get('test:transaction:1');
   $pipe->set('test:transaction:2', 2);
   $pipe->get('test:transaction:2');
});

dump($result);

array:4 [
  0 => true
  1 => "1"
  2 => true
  3 => "2"
]

PhpRedis docs: https://github.com/phpredis/phpredis#multi-exec-discard
Predis docs: https://github.com/nrk/predis#transactions

3. Raw Commands

Predis Style:

Redis::set('test:raw', 1);
Redis::executeRaw(['GET', 'test:raw']);

PhpRedis Style:

Redis::set('test:raw', 1);
Redis::rawCommand('GET', 'test:raw');

PhpRedis docs: https://github.com/phpredis/phpredis#rawcommand
Predis docs: https://github.com/nrk/predis#adding-new-commands

---

Also I added 3 tests for these methods.

[alibo]

It looks like the old version of phpredis doesn't have rawCommand method. I'm working on it...

[alibo]

The method rawCommand has been implemented in version 2.2.7 on Jan 18, 2015 (2 years ago!).
The preinstalled Redis extension for PHP 5.6 in Travis CI is very old.

I changed travis.yml and now it installs the latest version with pecl. If you think it may break something, I can remove method executeRaw. What do you think?

[lucasmichot]

Why do you reinstall Redis extension?
It is already installed: https://docs.travis-ci.com/user/languages/php#PHP-7.0

[alibo]

@lucasmichot you're right, but the problem is the installed version. for PHP 7 and 7.1, it's ok, but for PHP 5.6, it has an old version of phpredis extension (< 2.2.7).