minor #17889 [ci] clone with depth=1 to kill push-forced PRs (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[ci] clone with depth=1 to kill push-forced PRs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

As documented in https://github.com/travis-ci/travis-ci/issues/4575#issuecomment-125200308, setting a clone depth of 1 will make push-forced PRs to fail quickly so that our CI won't spend too much time on them.

Commits
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562d4d6 [ci] clone with depth=1 to kill push-forced PRs

.travis.yml

@@ -2,6 +2,9 @@ language: php
 
 sudo: false
 
+git:
+    depth: 1
+
 addons:
     apt_packages:
         - parallel

appveyor.yml

@@ -1,6 +1,5 @@
 build: false
-shallow_clone: true
-platform: x86
+clone_depth: 1
 clone_folder: c:\projects\symfony
 
 cache: