Support for Monolog 2

[boboudreau]

Monolog 2 removed all non-PSR3 calls including all add*() methods. By using these methods, this library maintains Monolog 1 compatibility, but does not break when using Monolog 2.

[boboudreau]

Failing tests are because this library still attempts to test against PHP 5.5, and doesn't specify which version of PHPUnit to use in the composer.json file. Not sure if this library is even maintained anymore, but I'd be willing to work with whoever to bring some of the testing up to date if there's desire to do so.

[boboudreau]

Travis (and the world, really) does not support PHP <= 5.5, so it's probably a good idea to remove that from the travis.yml file, here, too. The rest of the tests pass. I'll let this simmer for a bit and then will update travis.yml if we need a passing PR to make it into master.

[mfn]

See related issue I reported a bit before this PR => https://github.com/wa0x6e/php-resque-ex/issues/53

@wa0x6e are you still active?

[wa0x6e]

Let's go ahead and remove php 5.5 from travis, and I'll merge this PR

[mfn]

@boboudreau are you still up to finishing this PR? Otherwise I would be available too.

[wa0x6e]

Please remove the old php version from travis, so test can pass, and I'll merge it