stop building for 5.3, 5.4, 5.5 and 7.0

.travis.yml

@@ -3,27 +3,26 @@ language: php
 dist: precise
 
 php:
-    - 5.3
-    - 5.4
-    - 5.5
     - 5.6
-    - 7.0
     - 7.1
     - 7.2
     - 7.3
+    - nightly
 
 matrix:
     include:
-        - php: 7.0
+        - php: 7.2
           env: COVERAGE=1
-        - php: 5.3
-          env: COMPOSER_MEMORY_LIMIT=2G
         - php: 7.3
           env: DEPENDENCIES="--ignore-platform-reqs"
+        - php: nightly
+          env: DEPENDENCIES="--ignore-platform-reqs"
     exclude:
-        - php: 5.3
-        - php: 7.0
+        - php: 7.2
         - php: 7.3
+        - php: nightly
+    allow_failures:
+        - php: nightly
 
 cache:
     directories: