Travis: enable Composer cache

See:
- https://docs.travis-ci.com/user/caching/
- https://blog.wyrihaximus.net/2015/07/composer-cache-on-travis/

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: php
+cache:
+  directories:
+    - $HOME/.composer/cache
 php:
   - 7.0
   - 5.6
@@ -8,7 +11,7 @@ php:
   - 5.3
 install:
   - composer self-update
-  - composer install
+  - composer install --prefer-dist
 script:
   - make clean
   - make check_permissions

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [v0.8.1](https://github.com/shaarli/Shaarli/releases/tag/v0.8.1) - UNPUBLISHED
 ### Added
 - Add CHANGELOG.md to track the whole project's history
+- Enable Composer cache for Travis builds
 - Save the last edition date for shaares and use it in Atom/RSS feeds
 - Plugins:
     - Add an [Isso](https://posativ.org/isso/) plugin to enable user comments on permalinks