Remove .gitattributes, it breaks unit/behat test usage

Since it excludes behat.yml and phpunit.xml.dist,
and "composer install" uses github archives for stable versions,
you can't use projects installed this way for testing any more.
To make matters worse, the error you get on a "phpunit" run
is very confusing ('class SapphireTest not found').
Merge pull request #69 from chillu/pulls/gitattrs

Remove .gitattributes, it breaks unit/behat test usage