PHP 5.5 is the new minimum requirement

This is a decision annouced by @MarkBaker in Gitter to be able to
use `DateTimeInterface`.

Also remove deprecated composer option, and introduce caching for
faster CI tests

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 
 php:
-  - 5.4
   - 5.5
   - 5.6
   - 7.0
@@ -11,19 +10,20 @@ matrix:
   allow_failures:
     - php: hhvm
 
+cache:
+  directories:
+    - vendor
+    - $HOME/.composer/cache
+
 before_script:
   ## Packages
   - sudo apt-get -qq update > /dev/null
   ## Composer
   - composer self-update
-  - composer install --prefer-source --dev
-  - phpenv global "$TRAVIS_PHP_VERSION"
+  - composer install
 
 script:
   ## PHP_CodeSniffer
   - ./vendor/bin/phpcs --report-width=200 --report-summary  --report-full src/ unitTests/ --standard=PSR2 -n
   ## PHPUnit
   - ./vendor/bin/phpunit -c ./unitTests/
-
-notifications:
-  email: false