Replace individual linters with golangci-lint

Add .golangci.yml config file for golangci-lint. Simplify the Travis config and
Gopkg.toml because now there is only one linter to go get. Don't vendor
golangci-lint because dep fetches the wrong versions of golangci-lint's
dependencies.

.golangci.yml

@@ -0,0 +1,19 @@
+run:
+  deadline: 1h
+  tests: true
+
+linters:
+  enable:
+    - gocyclo
+    - gofmt
+    - golint
+
+linters-settings:
+  gocyclo:
+    min-complexity: 11
+
+  gofmt:
+    simplify: true
+
+  golint:
+    min-confidence: 0.25  # ignore missing package comment false positive

.travis.yml

@@ -9,24 +9,10 @@ install: true
 notifications:
   email: false
 
-# install code checkers
+# install linter
 before_script:
-  - cd vendor/golang.org/x/lint/golint
-  - go install
-  - cd -
-  - cd vendor/honnef.co/go/tools/cmd/megacheck
-  - go install
-  - cd -
-  - cd vendor/github.com/fzipp/gocyclo
-  - go install
-  - cd -
+  - go get -u github.com/golangci/golangci-lint/cmd/golangci-lint
 
 script:
-  - GO_FILES=$(find . -iname '*.go' -type f | grep -v /vendor/)
-  - test -z $(gofmt -s -l ${GO_FILES})
+  - golangci-lint --concurrency $(nproc) run
   - go test -v -race ./...
-  - go vet ./...
-  - megacheck ./...
-  # golint ./... doesn't ignore vendor/ for some dumb reason
-  - golint -set_exit_status $(go list ./... | grep -v /vendor/)
-  - gocyclo -over 10 ${GO_FILES}

Gopkg.toml

@@ -1,19 +1,8 @@
-required = [
-	"github.com/fzipp/gocyclo",
-	"golang.org/x/lint",
-	"golang.org/x/lint/golint",
-	"honnef.co/go/tools/cmd/megacheck",
-]
-
-[[constraint]]
-name = "github.com/fzipp/gocyclo"
-branch = "master"
-
 [[constraint]]
-name = "golang.org/x/lint"
-branch = "master"
+  name = "github.com/kr/pretty"
+  version = "^0.1.0"
 
 [prune]
-go-tests = true
-non-go = true
-unused-packages = true
+  go-tests = true
+  non-go = true
+  unused-packages = true