update composer to use symfony/var-dumper ^2.6 and ^3.0 || added travis support for lowest composer install

.travis.yml

@@ -9,12 +9,22 @@ php:
 
 sudo: false
 
+env:
+  global:
+    SETUP=basic
+
+matrix:
+  include:
+    - php: 5.6
+      env: SETUP=lowest
+
 cache:
   directories:
     - "$HOME/.composer/cache"
 
 before_script:
-  - composer install --no-interaction --prefer-dist
+  - if [[ "$SETUP" = "basic" ]]; then composer install --no-interaction --prefer-dist; fi
+  - if [[ "$SETUP" = "lowest" ]]; then travis_retry composer update --prefer-dist --no-interaction --prefer-lowest --prefer-stable; fi
   - if [ $TRAVIS_PHP_VERSION = "5.6" ]; then PHPUNIT_FLAGS="--coverage-clover build/logs/clover.xml"; fi
 
 script:

composer.json

@@ -17,7 +17,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8 || ^5.0",
         "mockery/mockery": "0.9.*",
-        "symfony/var-dumper": "~3.0"
+        "symfony/var-dumper": "^2.6 || ^3.0"
     },
     "suggest": {
         "symfony/var-dumper": "Pretty print complex values better with var-dumper available",