build: use matrix tests in CircleCI (#27)

* build: use matrix tests in CircleCI

Per https://circleci.com/blog/circleci-matrix-jobs/

* build: fix params

* build: fix YAML indent

* build: rename build -> tests

* build: Merge latest && matrix build

.circleci/config.yml

@@ -3,12 +3,9 @@ version: 2.1
 jobs:
   "test":
     parameters:
-      v:
+      version:
         type: string
         default: "latest"
-      latest:
-        type: boolean
-        default: false
       golint:
         type: boolean
         default: true
@@ -19,7 +16,7 @@ jobs:
         type: string
         default: ""
     docker:
-      - image: "circleci/golang:<< parameters.v >>"
+      - image: "circleci/golang:<< parameters.version >>"
     working_directory: /go/src/github.com/elithrar/admission-control
     environment:
       GO111MODULE: "on"
@@ -43,20 +40,20 @@ jobs:
       - run:
           name: "Run golint"
           command: >
-            if [ << parameters.latest >> = true ] && [ << parameters.golint >> = true ]; then
+            if [ << parameters.version >> = "latest" ] && [ << parameters.golint >> = true ]; then
               go get -u golang.org/x/lint/golint
               golint ./...
             fi
       - run:
           name: "Run gofmt"
           command: >
-            if [[ << parameters.latest >> = true ]]; then
+            if [[ << parameters.version >> = "latest" ]]; then
               diff -u <(echo -n) <(gofmt -d -e .)
             fi
       - run:
           name: "Run go vet"
           command: >
-            if [[ << parameters.latest >> = true ]]; then
+            if [[ << parameters.version >> = "latest" ]]; then
               go vet -v ./...
             fi
       - run:
@@ -66,7 +63,7 @@ jobs:
 
   "build-container":
     docker:
-      - image: docker:18
+      - image: docker:19
     working_directory: /go/src/github.com/elithrar/admission-control
     steps:
       - checkout
@@ -81,19 +78,10 @@ jobs:
             docker build -t elithrar/admissiond-example .
 
 workflows:
-  build:
+  tests:
     jobs:
       - test:
-          name: "latest"
-          v: "latest"
-          latest: true
-      - test:
-          name: "v1.15"
-          v: "1.15"
-      - test:
-          name: "v1.14"
-          v: "1.14"
-      - test:
-          name: "v1.13"
-          v: "1.13"
+          matrix:
+            parameters:
+              version: ["latest", "1.15", "1.14", "1.13"]
       - "build-container"