Ensures that packages used under 5.6 can run under 5.6

Install all deps always, but then downgrade if we are running under PHP
5.6.

.travis.yml

@@ -9,8 +9,9 @@ cache:
 
 env:
   global:
-    - COMPOSER_ARGS="--no-interaction --ignore-platform-reqs"
+    - COMPOSER_ARGS="--no-interaction"
     - COVERAGE_DEPS="satooshi/php-coveralls"
+    - LEGACY_DEPS="phpunit/phpunit doctrine/instantiator"
 
 matrix:
   include:
@@ -64,10 +65,11 @@ before_install:
   - travis_retry composer self-update
 
 install:
-  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS $COVERAGE_DEPS ; fi
+  - travis_retry composer install $COMPOSER_ARGS --ignore-platform-reqs
+  - if [[ $TRAVIS_PHP_VERSION =~ ^5.6 ]]; then travis_retry composer update $COMPOSER_ARGS --with-dependencies $LEGACY_DEPS ; fi
   - if [[ $DEPS == 'latest' ]]; then travis_retry composer update $COMPOSER_ARGS ; fi
   - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update --prefer-lowest --prefer-stable $COMPOSER_ARGS ; fi
-  - travis_retry composer install $COMPOSER_ARGS
+  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS $COVERAGE_DEPS ; fi
   - composer show
 
 script: