phpunit.xml.dist: Add coverage report generation

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>
Add php-coveralls to composer dev dependencies

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>
.travis.yml: Add Coveralls.io integration

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>