Test coverage for all packages and merge Travis builds (#781)

This halves the number of Travis builds that will run, while adding
coverage testing to the dnsutil package.

.travis.yml

@@ -1,21 +1,18 @@
 language: go
 sudo: false
+
 go:
   - 1.10.x
   - 1.11.x
   - tip
 
-env:
-  - TESTS="-race -v -bench=. -coverprofile=coverage.txt -covermode=atomic"
-  - TESTS="-race -v ./..."
-
 before_install:
   # don't use the miekg/dns when testing forks
   - mkdir -p $GOPATH/src/github.com/miekg
   - ln -s $TRAVIS_BUILD_DIR $GOPATH/src/github.com/miekg/ || true
 
 script:
-  - go test $TESTS
+  - go test -race -v -bench=. -coverprofile=coverage.txt -covermode=atomic ./...
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)