Test both composer install / composer update

Closes #628

.travis.yml

 language: php
 php:
-- 5.5
-- 5.6
-- 7.0
-- 7.1
-- 7.2
-- 7.3
-- hhvm
+  - 5.5
+  - 5.6
+  - 7.0
+  - 7.1
+  - 7.2
+  - 7.3
+  - hhvm
+
+env:
+  - COMMAND="composer install"
+  - COMMAND="composer update"
+
 matrix:
   allow_failures:
-  - php: hhvm
+    - php: hhvm
+
 before_script:
-- composer update
-- if [[ "$TRAVIS_PHP_VERSION" == "7.0" ]]; then composer require --dev vimeo/psalm:0.3.92; fi
-- if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then composer require --dev php-coveralls/php-coveralls; fi
+  - if [[ ${COMMAND} == "composer install" ]]; then ${COMMAND}; else composer update; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == "7.0" ]]; then composer require --dev vimeo/psalm:0.3.92; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then composer require --dev php-coveralls/php-coveralls; fi
+
 script:
-- bin/check-syntax.sh
-- if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then php vendor/phpunit/phpunit/phpunit; else php vendor/phpunit/phpunit/phpunit --no-coverage; fi
-- if [[ "$TRAVIS_PHP_VERSION" == "7.0" ]]; then vendor/bin/psalm; fi
+  - bin/check-syntax.sh
+  - if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then php vendor/phpunit/phpunit/phpunit; else php vendor/phpunit/phpunit/phpunit --no-coverage; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == "7.0" ]]; then vendor/bin/psalm; fi
+
 after_success:
-- if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then php vendor/bin/php-coveralls -v; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then php vendor/bin/php-coveralls -v; fi
+
 notifications:
   slack:
     secure: h5YiO7tAk5bzAQBkUZ3crnOFAOBkNqHp0nc2cxvGauogVfCLE9K8bME512uTeu9QWMht2D30uyTOHFKQ7sPEW4omHNF4qYl4p9L037oQFfEwZPBzJFo9QUzviT3mvKHTN4HSQOkq/08ZoMhh1i3p/OhyFS56J28IDJWDqGZkEUE=