remove composer.lock, test on travis with lowest deps

Author: keradus

.gitignore

@@ -5,3 +5,4 @@ coverage
 .buildpath
 .project
 .settings
+composer.lock

.travis.yml

@@ -12,10 +12,9 @@ matrix:
     allow_failures:
         - php: hhvm
 
-before_script:
+install:
   - sudo apt-get -y install pypy python-sphinx graphviz
-  - composer selfupdate
-  - composer install --prefer-source
+  - travis_retry composer update --no-interaction
 
 script:
   - cd docs && make linkcheck && cd ..