Enhancement: Cache dependencies installed with Composer between builds

Author: localheinz

.travis.yml

@@ -16,11 +16,15 @@ matrix:
   allow_failures:
     - php: 7
 
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 before_install:
   - composer self-update
 
 install:
-  - composer install --no-interaction --prefer-source
+  - composer install
 
 script:
   - vendor/bin/phpdoc -d src -t docs-api