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

This reverts commit 08bb9a4.

Reverting this change won't harm but it's possible that it solves
"Cannot find a merge base between danger_base and danger_head" error
from Danger.

Author: php-coder

.travis.yml

@@ -63,8 +63,3 @@ 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