diff --git a/.github/rm-invalid-lowest-lock-files.php b/.github/rm-invalid-lowest-lock-files.php
index 81f58ccdd4..2b037519cf 100644
--- a/.github/rm-invalid-lowest-lock-files.php
+++ b/.github/rm-invalid-lowest-lock-files.php
@@ -96,7 +96,7 @@ foreach ($composerJsons as list($dir, $lockedPackages)) {
     }
 }
 
-if (!$referencedCommits || (isset($_SERVER['TRAVIS_PULL_REQUEST']) && 'false' === $_SERVER['TRAVIS_PULL_REQUEST'])) {
+if (!$referencedCommits || (isset($_SERVER['TRAVIS_PULL_REQUEST']) && 'false' !== $_SERVER['TRAVIS_PULL_REQUEST'])) {
     // cached commits cannot be stale for PRs
     return;
 }
@@ -142,8 +142,10 @@ foreach ($referencedCommits as $name => $dirsByCommit) {
 foreach ($referencedCommits as $name => $dirsByCommit) {
     foreach ($dirsByCommit as $dirs) {
         foreach ($dirs as $dir) {
-            echo "$dir/composer.lock references old commit for $name.\n";
-            @unlink($dir.'/composer.lock');
+            if (file_exists($dir.'/composer.lock')) {
+                echo "$dir/composer.lock references old commit for $name.\n";
+                @unlink($dir.'/composer.lock');
+            }
         }
     }
 }
diff --git a/.travis.yml b/.travis.yml
index 7f69934fbc..68b36579f4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -216,7 +216,7 @@ install:
       else
           export SYMFONY_REQUIRE=">=$SYMFONY_VERSION"
       fi
-      composer global require symfony/flex dev-master
+      composer global require --no-progress --no-scripts --no-plugins symfony/flex dev-master
 
     - |
       # Legacy tests are skipped when deps=high and when the current branch version has not the same major version number than the next one
diff --git a/appveyor.yml b/appveyor.yml
index c4747644f0..45373a9358 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -52,7 +52,7 @@ install:
     - IF NOT EXIST composer.phar (appveyor DownloadFile https://github.com/composer/composer/releases/download/1.7.1/composer.phar)
     - php composer.phar self-update
     - copy /Y .composer\* %APPDATA%\Composer\
-    - php composer.phar global require --no-progress symfony/flex dev-master
+    - php composer.phar global require --no-progress --no-scripts --no-plugins symfony/flex dev-master
     - php .github/build-packages.php "HEAD^" src\Symfony\Bridge\PhpUnit
     - IF %APPVEYOR_REPO_BRANCH%==master (SET COMPOSER_ROOT_VERSION=dev-master) ELSE (SET COMPOSER_ROOT_VERSION=%APPVEYOR_REPO_BRANCH%.x-dev)
     - php composer.phar config platform.php 5.5.9