Stop versioning composer.lock

`composer.lock` should only be versioned for projects, not for libraries.

Author: oliverklee

.gitignore

@@ -1 +1,2 @@
-vendor/
+/composer.lock
+/vendor/

.travis.yml

@@ -17,6 +17,6 @@ matrix:
             php: "5.3"
             dist: precise
             sudo: false
-before_script: rm composer.lock && composer install
+before_script: composer install
 script: ./vendor/bin/phpunit --coverage-clover build/coverage/xml
 after_script: ./vendor/bin/codacycoverage clover build/coverage/xml