Run test coverage only on PHP 7

Previously it runs on PHP 5.6 and PHP 7.2 (probably by mistake)

Author: michalbundyra

.travis.yml

@@ -20,7 +20,6 @@ matrix:
     - php: 5.6
       env:
         - DEPS=locked
-        - TEST_COVERAGE=true
     - php: 5.6
       env:
         - DEPS=latest
@@ -31,6 +30,7 @@ matrix:
       env:
         - DEPS=locked
         - CHECK_CS=true
+        - TEST_COVERAGE=true
     - php: 7
       env:
         - DEPS=latest
@@ -49,7 +49,6 @@ matrix:
     - php: 7.2
       env:
         - DEPS=locked
-        - TEST_COVERAGE=true
     - php: 7.2
       env:
         - DEPS=latest