travis: Filter out Travis-related envvars

This is causing us to attempt to have a home directory in /home/travis
instead of the user's real homedir.

.travis.yml

@@ -25,7 +25,7 @@ env:
 # Trusty (aka 14.04) is way way too old, so run in docker...
 script:
   - docker pull cdecker/lightning-ci:${ARCH}bit | tee
-  - env | grep -E '^[A-Z_]+\=' | tee /tmp/envlist
+  - env | grep -E '^[A-Z_]+\=' | grep -vi travis | tee /tmp/envlist
   - echo SLOW_MACHINE=1 >> /tmp/envlist
   - docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit ./configure CC=${COMPILER:-gcc}
   - $SOURCE_CHECK_ONLY || docker run --rm=true --env-file=/tmp/envlist -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make -j3