Remove PHP 5.3 from Travis matrix, add PHP7.1

Travis no longer supports testing against pre-installed PHP5.3, and we
don't have to either. Updating the build matric so new tests can pass
(see #78 (comment))

.travis.yml

@@ -6,17 +6,15 @@ notifications:
   on_success: never
   on_failure: change
 
-php:
-  - 5.3
-  - 5.5
-
 env:
   - WP_VERSION=latest WP_MULTISITE=0
 
 matrix:
   include:
-    - php: 5.3
+    - php: 5.6
       env: WP_VERSION=latest WP_MULTISITE=1
+    - php: 7.1
+	  env: WP_VERSION=latest WP_MULTISITE=0
 
 cache:
   - composer
@@ -25,7 +23,7 @@ cache:
 before_script:
   - composer install
   - npm install
-  - bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION 
+  - bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
 
 script:
   - grunt phpcs