Update TravisCI configuration

.travis.yml

@@ -2,13 +2,22 @@ language: php
 
 php:
   - 7.0
+  - 7.0snapshot
   - 7.1
-  - nightly
+  - 7.1snapshot
+  - master
+
+matrix:
+  allow_failures:
+    - php: master
+  fast_finish: true
 
 env:
   matrix:
-    - DRIVER="xdebug"
-    - DRIVER="phpdbg"
+    - DRIVER="xdebug" DEPENDENCIES="high"
+    - DRIVER="phpdbg" DEPENDENCIES="high"
+    - DRIVER="xdebug" DEPENDENCIES="low"
+    - DRIVER="phpdbg" DEPENDENCIES="low"
 
 sudo: false
 
@@ -17,7 +26,8 @@ before_install:
   - composer clear-cache
 
 install:
-  - travis_retry composer update --no-interaction --no-ansi --no-progress --no-suggest --optimize-autoloader --prefer-stable
+  - if [[ "$DEPENDENCIES" = 'high' ]]; then travis_retry composer update --no-interaction --no-ansi --no-progress --no-suggest --optimize-autoloader --prefer-stable; fi
+  - if [[ "$DEPENDENCIES" = 'low' ]]; then travis_retry composer update --no-interaction --no-ansi --no-progress --no-suggest --optimize-autoloader --prefer-stable --prefer-lowest; fi
 
 script:
   - if [[ "$DRIVER" = 'phpdbg' ]]; then phpdbg -qrr vendor/bin/phpunit; fi