build: install btcd binary within Travis container

This commit modifies the existing Travis configuration to also install
the btcd binary within the container’s PATH.

This change is necessary in order for tests written against the new
rpctest package to work properly within the Travis testing environment.

.travis.yml

@@ -6,6 +6,7 @@ sudo: false
 install:
   - go get -v github.com/Masterminds/glide
   - glide install
+  - go install . ./cmd/...
   - go get -v golang.org/x/tools/cmd/cover
   - go get -v github.com/golang/lint/golint
 script: