Remove composer.lock from version control

This is our new policy.

Author: greg0ire

.gitignore

@@ -1,3 +1,4 @@
 /vendor/
+/composer.lock
 /.phpcs-cache
 

.travis.yml

@@ -17,7 +17,6 @@ before_install:
   - travis_retry composer self-update
 
 install:
-  - rm composer.lock
   - travis_retry composer update --prefer-dist
 
 script:
@@ -28,7 +27,6 @@ jobs:
     - stage: Test
       env: DEPENDENCIES=low
       install:
-        - rm composer.lock
         - travis_retry composer update --prefer-dist --prefer-lowest
 
     - stage: Test
@@ -44,5 +42,5 @@ jobs:
 
     - stage: Code Quality
       env: STATIC_ANALYSIS
-      install: travis_retry composer install --prefer-dist
+      install: travis_retry composer update --prefer-dist
       script: vendor/bin/phpstan analyse