Pass through BOULDER_CONFIG in .travis.yml (letsencrypt#1954)

Moving to Docker meant that we weren't passing through the BOULDER_CONFIG
variable properly, which meant we weren't testing the boulder-config-next.json
configuration. That allowed letsencrypt#1948
to pass unnoticed.

Credit to @benileo for asking the question of why that issue wasn't caught in
testing, which led to this fix. Thanks for the attention to detail!

.travis.yml

@@ -49,4 +49,4 @@ install:
   - docker-compose build
 
 script:
-  - docker-compose run -e RUN="${RUN}" -e TRAVIS="${TRAVIS}" -e TRAVIS_COMMIT="${TRAVIS_COMMIT}" -e TRAVIS_PULL_REQUEST="${TRAVIS_PULL_REQUEST}" -e TRAVIS_PULL_REQUEST="${TRAVIS_PULL_REQUEST}" -e TRAVIS_JOB_ID="${TRAVIS_JOB_ID}" -e COVERALLS_TOKEN="${COVERALLS_TOKEN}" boulder  ./test.sh
+  - docker-compose run -e BOULDER_CONFIG="${BOULDER_CONFIG}" -e RUN="${RUN}" -e TRAVIS="${TRAVIS}" -e TRAVIS_COMMIT="${TRAVIS_COMMIT}" -e TRAVIS_PULL_REQUEST="${TRAVIS_PULL_REQUEST}" -e TRAVIS_PULL_REQUEST="${TRAVIS_PULL_REQUEST}" -e TRAVIS_JOB_ID="${TRAVIS_JOB_ID}" -e COVERALLS_TOKEN="${COVERALLS_TOKEN}" boulder  ./test.sh

test/startservers.py

@@ -11,8 +11,8 @@
 import threading
 import time
 
-default_config = os.environ.get('BOULDER_CONFIG')
-if default_config is None:
+default_config = os.environ.get('BOULDER_CONFIG', '')
+if default_config == '':
     default_config = 'test/boulder-config.json'
 processes = []