dev/2.1.0 - #15

- Removes the `SimpleCache`
- adds global TTL in all clients
- improves the FileClient
- renames `guard_cache_key` -> `cache_key_check`
- Redis dual-cache stores the second item with suffix based on the serializer type

- supports DateTimeInterface the `cache_ttl()`
- optimizes the TTL calculation for all clients

- ensures NULL

- removed NullClient and replaced by MemoryClient as default
- using the "cache/integration-tests" suite to verify the PSR-16
- improved all cache functions
- cleaned up unit tests in favor of integration test package

- calls cache_ttl() in MultipleTrait::setMultiple()

- refactor in all clients
- renamed cache_ley_check() -> verify_key()
- renamed cache_ttl() -> verify_key()
- attempt to fix the unusual flaky tests behavior when JSON serializer is used

- renaming multi* -> internalMulti*

- internalMulti* lowered from protected to private
- adds some docs

- throws exception on unknown cache client
- improves the ConfigFactory
-

- improve MemoryClient
- cleanup referenced variables in FileClient
- improve some configuration constructors
- remove "options" for Predis (erratic behavior somewhere)
- unit test improvements

- increase PHP min version

- updates the PHP matrix
- uses Codacy
- other improvements

- PHP min. requirement is 7.2

� Conflicts:
�	.travis.yml
�	README.md
�	Tests/Client/FileClientTest.php
�	Tests/Client/MemoryClientTest.php
�	Tests/FileTest.php
�	Tests/Integration/FileClientTest.php
�	Tests/Integration/MemcachedClientTest.php
�	Tests/Integration/PredisClientTest.php
�	Tests/Integration/PredisJsonClientTest.php
�	Tests/Integration/RedisClientTest.php
�	Tests/Integration/RedisJsonClientTest.php
�	Tests/Integration/SimpleCacheIntegrationTrait.php
�	Tests/PredisConnectionTest.php
�	Tests/PredisWithJsonSerializerTest.php
�	Tests/RedisConnectionTest.php
�	Tests/RedisWithOnlyJsonSerializerTest.php
�	VERSION
�	composer.json