Update the Omnibus package in Dockerfile

From 7.5.2 to 7.5.3.
Signed-off-by: kfei <kfei@kfei.net>

docker/Dockerfile

@@ -11,7 +11,7 @@ RUN apt-get update -q \
 # If the Omnibus package version below is outdated please contribute a merge request to update it.
 # If you run GitLab Enterprise Edition point it to a location where you have downloaded it.
 RUN TMP_FILE=$(mktemp); \
-    wget -q -O $TMP_FILE https://downloads-packages.s3.amazonaws.com/ubuntu-14.04/gitlab_7.5.2-omnibus.5.2.1.ci-1_amd64.deb \
+    wget -q -O $TMP_FILE https://downloads-packages.s3.amazonaws.com/ubuntu-14.04/gitlab_7.5.3-omnibus.5.2.1.ci-1_amd64.deb \
     && dpkg -i $TMP_FILE \
     && rm -f $TMP_FILE