Move arm64 job to cron

As we're branching soon, reduce the number of jobs we run on each
commit, so that Travis can keep up with its role as the fast
feedback build.

Author: nikic

.travis.yml

@@ -74,6 +74,7 @@ jobs:
           arch: amd64
         - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 ARM64=1
           arch: arm64
+          if: type = cron
         - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 S390X=1
           arch: s390x
           if: type = cron