Introduce Travis matrix includes

The goal is to test the library against multiple versions of Symfony and with lowest dependencies.

.travis.yml

@@ -8,6 +8,7 @@ php:
 # Commands to be run before your environment runs.
 before_script:
   - composer self-update
+  - if [ "$SYMFONY" != "" ]; then composer require "symfony/symfony:${SYMFONY}" --no-update; fi;
   - composer update --prefer-source --no-interaction
 
 
@@ -28,6 +29,15 @@ matrix:
 #    - php: 7
 #    - php: hhvm
   fast_finish: true
+  include:
+    - php: '7.0'
+      env: COMPOSER_FLAGS="--prefer-lowest"
+    - php: '7.0'
+      env: SYMFONY=2.8.*
+    - php: '7.0'
+      env: SYMFONY=3.0.*
+    - php: '7.0'
+      env: SYMFONY=3.1.*
 
 # Customize when the notification emails are sent.
 notifications: