Run Travis-CI on nightly PHP 7 build for 2.1 and above. Closes #2

Signed-off-by: crynobone <crynobone@gmail.com>

.travis.yml

@@ -5,18 +5,20 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - 7.0
   - hhvm
 
 before_script:
   - travis_retry composer self-update
-  - travis_retry composer require satooshi/php-coveralls:~0.6 --prefer-source --no-interaction --dev
+  - if [[ "$TRAVIS_PHP_VERSION" == *5.6* ]]; then travis_retry composer require "satooshi/php-coveralls:~0.6.1" "symfony/yaml=~2.0" --prefer-source --no-interaction --dev; else travis_retry composer install --prefer-source --no-interaction; fi
 
 script: phpunit -c phpunit.xml --coverage-text
 
 after_script:
-  - if [[ "$TRAVIS_PHP_VERSION" != *hhvm* ]]; then php vendor/bin/coveralls -v; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == *5.6* ]]; then php vendor/bin/coveralls -v; fi
 
 matrix:
   allow_failures:
+    - php: 7.0
     - php: hhvm
   fast_finish: true