Travis: switch to Ubuntu Trusty build environment

Relates to https://github.com/shaarli/Shaarli/issues/970
Relates to https://github.com/shaarli/Shaarli/pull/912

See:
- https://docs.travis-ci.com/user/reference/trusty/
- https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming

Added:
- print available locales before running tests

Removed:
- do not install extra language packs

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

.travis.yml

@@ -1,12 +1,6 @@
 sudo: false
-dist: precise
+dist: trusty
 language: php
-addons:
-  apt:
-    packages:
-      - locales
-      - language-pack-de
-      - language-pack-fr
 cache:
   directories:
     - $HOME/.composer/cache
@@ -19,6 +13,7 @@ install:
   - composer self-update
   - composer install --prefer-dist
 script:
+  - locale -a
   - make clean
   - make check_permissions
   - make all_tests