.travis.yml: clone repository with a depth of 1 commit.

No functional changes.

Author: php-coder

.travis.yml

@@ -54,3 +54,8 @@ notifications:
 
 # We don't want to run `mvn install -DskipTests=true` for downloading project's dependencies
 install: true
+
+# Clone repository with a depth of 1 commit.
+# https://docs.travis-ci.com/user/customizing-the-build/#Git-Clone-Depth
+git:
+  depth: 1 # default: 50