Disable coveralls on all but the PHP 7.2 build

.travis.yml

@@ -1,10 +1,12 @@
 language: php
 
-php:
-  - '5.6'
-  - '7.0'
-  - '7.1'
-  - '7.2'
+matrix:
+  include:
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
+    - php: 7.2
+      env: COVERALLS=true
 
 cache:
   directories:
@@ -15,6 +17,7 @@ before_install:
     - export INI=~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
     # disable default memory limit
     - echo memory_limit = 2G >> $INI
+    - if [[ $COVERALLS = "" ]]; then phpenv config-rm xdebug.ini; fi
 
 install:
   - composer --prefer-source install