Restructure the TravisCI script

The script should only run the tests; the base Docker image pull, as
well as the CI image build, should be performed in the preparation
stage.

.travis.yml

@@ -21,8 +21,10 @@ services:
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker pull ebassi/epoxyci ; fi
 
-script:
+before_script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo FROM ebassi/epoxyci > Dockerfile ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo ADD . /root >> Dockerfile ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker build -t withgit . ; fi
+
+script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && CC=$CC ./epoxy-run-tests.sh" ; fi