Adopts most recent composer install strategy

- Ignore platform requirements on initial install
- For PHP 5.6, update to use lower phpunit version (and its dependencies)
- Specify coverage dependencies via env argument

.travis.yml

@@ -9,6 +9,8 @@ cache:
 env:
   global:
     - COMPOSER_ARGS="--no-interaction"
+    - COVERAGE_DEPS="satooshi/php-coveralls"
+    - LEGACY_DEPS="phpunit/phpunit"
 
 matrix:
   include:
@@ -59,10 +61,11 @@ before_install:
   - if [[ $TEST_COVERAGE != 'true' && "$(php --version | grep xdebug -ci)" -ge 1 ]]; then phpenv config-rm xdebug.ini ; fi
 
 install:
+  - 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
-  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS satooshi/php-coveralls:^1.0 ; fi
-  - travis_retry composer install $COMPOSER_ARGS
+  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS $COVERAGE_DEPS ; fi
   - stty cols 120 && composer show
 
 script: