Allow usage with Monolog 2

Additionally:
- the processors now implement ProcessorInterface
- PHPUnit dependency updated to ^8.5 (^9.5 requires additional changes)
- Travis configured to build with currently supported PHP versions

.gitignore

@@ -1,3 +1,5 @@
 .idea/
+.phpunit.result.cache
 vendor/
 atlassian-ide-plugin.xml
+composer.lock

.travis.yml

@@ -1,7 +1,8 @@
 language: php
 php:
-  - '7.1'
-  - '7.2'
+  - '7.4'
+  - '8.0'
+  - '8.1'
 
 install:
   - composer install --prefer-dist

composer.json

@@ -6,10 +6,10 @@
   "type": "library",
   "license": "MIT",
   "require": {
-    "monolog/monolog": "^1.17"
+    "monolog/monolog": "^1.17|^2.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^4.8"
+    "phpunit/phpunit": "^8.5"
   },
   "autoload": {
     "psr-4": {"Egeniq\\Monolog\\Gdpr\\": "src/"}