ci: added gitlab CI configuration

which runs the following:

- standard python build with `setup.py sdist bdist_wheel`
- debian builds with `dpkg-buildpackage`
- el7 build with `rpmbuild`

.gitlab-ci.yml

+stages:
+  - build
+
+# -- build ------------------
+
+.build: &build
+  stage: build
+  after_script:
+    - ls -l dist/
+  artifacts:
+    expire_in: 3h
+    paths:
+      - dist
+
+build:tarball:
+  <<: *build
+  image: python:3.6
+  script:
+    - python setup.py sdist bdist_wheel
+
+build:el7:
+  <<: *build
+  image: ligo/base:el7
+  before_script:
+    - yum install -yq
+          rpm-build
+          epel-rpm-macros
+          python-rpm-macros
+          python3-rpm-macros
+          python-setuptools
+          python34-setuptools
+  script:
+    - python setup.py sdist
+    - rpmbuild -tb dist/ligo-gracedb*.tar.gz
+    - mv -v ~/rpmbuild/RPMS/*/python*-ligo-gracedb-*.rpm dist/
+    - rm -rf dist/*.tar.gz
+
+.build:debian: &build_debian
+  <<: *build
+  before_script:
+    - apt-get update -yqq
+    - apt-get install -yq
+          dpkg-dev
+          debhelper
+          dh-python
+          python-all-dev
+          python3-all-dev
+          python-setuptools
+          python3-setuptools
+          python-six
+          python3-six
+          help2man
+  script:
+    - pushd .
+    - python setup.py sdist
+    - version=$(python setup.py --version)
+    - cd dist
+    - tar -xf ligo-gracedb-${version}.tar.gz
+    - cd ligo-gracedb-${version}
+    - dpkg-buildpackage -us -uc -b
+    - popd
+    - rm -rf dist/ligo-gracedb*
+
+build:debian:jessie:
+  <<: *build_debian
+  image: debian:jessie
+
+build:debian:stretch:
+  <<: *build_debian
+  image: debian:stretch