Add composer test script

[MaxGfeller]

Composer gives us the possibility to define scripts in the composer.json file: Documentation on getcomposer.org

Most libraries now define a test script which makes running unit tests as easy as executing composer test.

Here is more on the topic:

• http://maxgfeller.com/blog/2014/07/22/composer-test/
• https://pantheon.io/blog/writing-composer-scripts

[coveralls]

Coverage remained the same at 96.341% when pulling a19fc9d on MaxGfeller:add-test-script into 4f44e7a on zumba:master.

[jrbasso]

Cool, thanks!