build: force 'go test' to build+test packages serially

This commit fixes some flakiness exhibited within the tests on Travis
due to the default behavior of the `go test` command to execute tests
amongst packages in parallel. Since many tests use the `rpctest`
package from `btcd`, many instances of `btcd` would be started at the
same time, with only one being able to grab the port and fully start
up. By forcing tests to be executed serially, this behavior should be
patched.

One downside is that builds on Travis will take longer. Therefore, this
may be  a temporary fix a more fundamental fix within the `rpctest`
package is implemented.

.travis.yml

@@ -10,4 +10,4 @@ install:
   - go install -v . # Required for the integration tests.
 script:
   - export PATH=$PATH:$HOME/gopath/bin
-  - go test -v $(go list ./... | grep -v '/vendor/')
+  - go test -p 1 -v $(go list ./... | grep -v '/vendor/')