Only run coverage on PHP 5.6 + PHP 7.2

Fixes #251

.travis.yml

@@ -2,20 +2,29 @@ sudo: false
 
 language: php
 
+env:
+  global:
+    - COVERAGE_FLAGS=" --coverage-text --coverage-clover build/logs/clover.xml"
+
 matrix:
     fast_finish: true
     include:
         # Need to use Precise for PHP 5.3
         - php: 5.3
-          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE_FLAGS=""
           dist: precise
         - php: 5.4
+          env: COVERAGE_FLAGS=""
         - php: 5.5
+          env: COVERAGE_FLAGS=""
         - php: 5.6
         - php: 7.0
+          env: COVERAGE_FLAGS=""
         - php: 7.1
+          env: COVERAGE_FLAGS=""
         - php: 7.2
         - php: hhvm
+          env: COVERAGE_FLAGS=""
 
 before_install:
  - travis_retry wget https://scrutinizer-ci.com/ocular.phar
@@ -26,7 +35,7 @@ install:
 
 script:
   - composer validate --strict
-  - ./vendor/bin/phpunit --coverage-text --coverage-clover build/logs/clover.xml
+  - ./vendor/bin/phpunit $COVERAGE_FLAGS
 
 after_success:
   - "[[ -f vendor/bin/php-coveralls ]] && COVERALLS_PATH=vendor/bin/php-coveralls || COVERALLS_PATH=vendor/bin/coveralls"