Fix sdist builder image

Fallout from https://github.com/ansible/awx/pull/982

installer/image_build/files/Dockerfile.sdist

@@ -1,11 +1,12 @@
 FROM centos:7
 
 RUN yum install -y epel-release
+RUN yum install -y https://centos7.iuscommunity.org/ius-release.rpm
 
 RUN yum install -y bzip2 \
     gcc-c++ \
     gettext \
-    git \
+    git2u-core \
     make \
     python \
     python-pip

installer/image_build/tasks/main.yml

@@ -1,6 +1,6 @@
 ---
 - name: Get Version from checkout if not provided
-  shell: "git describe --long | sed 's/\\-g.*//' | sed 's/\\-/\\./'"
+  shell: "git describe --long --first-parent | sed 's/\\-g.*//' | sed 's/\\-/\\./'"
   delegate_to: localhost
   register: awx_version_command
   when: awx_version is not defined