Use strings for PHP versions

Otherwise it's converted to number in YML and can cause strange results
(PHP 7.0 becomes 7 and it doesn't properly compare).

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -56,13 +56,13 @@ after_script:
 
 matrix:
   allow_failures:
-    - php: 5.6
+    - php: "5.6"
       env: PHPUNIT_ARGS="--group selenium" SELENIUM="yes" CI_MODE="selenium"
   fast_finish: true
   include:
-    - php: 5.6
+    - php: "5.6"
       env: PHPUNIT_ARGS="--group selenium" SELENIUM="yes" CI_MODE="selenium"
-    - php: 7.0
+    - php: "7.0"
       env: CI_MODE="release"
 cache:
   directories: