TravisCI: Disable race detector.
TravisCI: Disable race detector.

Race-enabled testing is an order of magnitude slower. This causes issues
for pull requests which add more tests, due to elapsed timeouts on the
CI infrastructure, and it is more desirable to have a comprehensive test
suite running without -race than few tests with it.

To test with -race locally, just run go test -race as usual, or (better)
build and run a race-enabled binary.

decred/dcrd,decred/dcrd,jrick/dcrd,jrick/dcrd