Drop support of old PHP versions

Author: xgin

.travis.yml

@@ -13,44 +13,15 @@ env:
 
 matrix:
   include:
-    - php: 5.6
-      env:
-        - DEPS=lowest
-    - php: 5.6
-      env:
-        - DEPS=locked
-        - LEGACY_DEPS="phpunit/phpunit"
-    - php: 5.6
-      env:
-        - DEPS=latest
-    - php: 7.0
-      env:
-        - DEPS=lowest
-    - php: 7.0
-      env:
-        - DEPS=locked
-        - LEGACY_DEPS="phpunit/phpunit"
-    - php: 7.0
-      env:
-        - DEPS=latest
-    - php: 7.1
+    - php: 7.2
       env:
         - DEPS=lowest
-    - php: 7.1
+    - php: 7.3
       env:
         - DEPS=locked
         - CS_CHECK=true
         - TEST_COVERAGE=true
-    - php: 7.1
-      env:
-        - DEPS=latest
-    - php: 7.2
-      env:
-        - DEPS=lowest
-    - php: 7.2
-      env:
-        - DEPS=locked
-    - php: 7.2
+    - php: 7.3
       env:
         - DEPS=latest