Internal-Security Finding #3: Set up merge-blocking CI on the repo

[defuse]

On the zcashd repo we block merges to master on the CI tests passing. We should do the same for all of our repos containing supported code. For lightwalletd, Travis-CI is good and easy enough, we probably don't need a whole buildbot setup, but I'll leave that up to dev-infra to decide.

The CI should test both 64-bit and 32-bit platforms to catch bugs like the one described in #28.