Up CI to use pre-built docker test environment

`galadirith/lvcnrpy:v1.0.0` already has all test dependencies to
dramatically reduce test suite run time on Gitlab CI.

.gitlab-ci.yml

-# Setup Python test environment with cache as Duncan does
-cache:
-  paths:
-    - apt-cache/
-    - pip-cache/
+image: galadirith/lvcnrpy:v1.0.0
 
-# Setup Python test environment
+# Setup `lvcnrpy` test environment
 before_script:
-  # Configure apt-get
-  - export APT_CACHE_DIR=`pwd`/apt-cache
-  - export PIP_CACHE_DIR=`pwd`/pip-cache
-  - mkdir -pv $APT_CACHE_DIR $PIP_CACHE_DIR
-  # Trust maintainer of lscsoft-archive-keyring package
-  - gpg --keyserver keys.gnupg.net --recv-keys CE050D236DB6FA3F
-  - gpg -a --export CE050D236DB6FA3F | apt-key add -
-  # tdd LSCSoft Debian Jessie archive
-  - echo deb http://software.ligo.org/lscsoft/debian jessie contrib >> /etc/apt/sources.list
-  - apt-get -o Dir::Cache::Archives="$APT_CACHE_DIR" -yqq update
-  - apt-get -o Dir::Cache::Archives="$APT_CACHE_DIR" -yqq install lscsoft-archive-keyring
-  - apt-get -o Dir::Cache::Archives="$APT_CACHE_DIR" -yqq update
-  # Install packages to setup Python test environment
-  - >
-    apt-get -o Dir::Cache::Archives="$APT_CACHE_DIR" -yqq install
-    wget
-    pkg-config
-    python
-    python-dev
-    python-pip
-    python-numpy
-    python-scipy
-    python-matplotlib
-    python-h5py
-    python-pytest
   # Define LVCNR_DATADIR (only needs to exist, not used for tests)
   - export LVCNR_DATADIR=`pwd`/lvc_nr
   - mkdir -pv $LVCNR_DATADIR
   # Install local `lvcnrpy` using pip
-  - pip install  --download-cache="$PIP_CACHE_DIR" .
+  - pip install .
 
 # Run `lvcnrpy` tests
 test: