TASK: Remove travis-ci configuration

Resolves #26

.travis.yml

-language: php
-
-cache:
-  directories:
-    - $HOME/.composer/cache
-
-php:
-  - '5.3'
-  - '5.4'
-  - '5.5'
-  - '5.6'
-  - '7.0'
-  - '7.1'
-
-install:
-  - 'composer install --prefer-dist'
-
-script:
-  - 'php vendor/bin/phpunit -c phpunit.xml.dist'