Removes tests for PHP 5.5

Why:

* PHP 5.5 is deprecated now and according to @MWOP we can drop support
  for 5.5 for the next minor release.

Author: heiglandreas

Diff for: .travis.yml

@@ -31,16 +31,6 @@ env:
 
 matrix:
   include:
-    - php: 5.5
-      env:
-        - DEPS=lowest
-    - php: 5.5
-      env:
-        - DEPS=locked
-        - CHECK_CS=true
-    - php: 5.5
-      env:
-        - DEPS=latest
     - php: 5.6
       env:
         - DEPS=lowest
@@ -61,6 +51,7 @@ matrix:
     - php: 7
       env:
         - TEST_COVERAGE=true
+        - CHECK_CS=true
         - DEPS=latest
     - php: 7.1
       env: