Starts testing with ICU 67.1

This is the last step in getting rust_icu tested with ICU 67.1  There
are a few pull requests in flight that preempt this one, such as google#75,
but once those are commited, this can go in. (And should pass.)

Fixes google#76.

.travis.yml

@@ -8,6 +8,7 @@ env:
   - DOCKER_TEST_ENV=rust_icu_testenv-64
   - DOCKER_TEST_ENV=rust_icu_testenv-65
   - DOCKER_TEST_ENV=rust_icu_testenv-66
+  - DOCKER_TEST_ENV=rust_icu_testenv-67
 
 script:
   - make docker-test