Support Drone environment variables

Drone used to support the CI_ environment variables, but they stopped
doing so.

Environment reference for Drone 0.{8,9}: https://0-8-0.docs.drone.io/environment-reference/

Environment reference for Drone 1.0 (upcoming release): https://docs.drone.io/reference/environ/

gitinfo.go

@@ -106,7 +106,7 @@ func collectGitInfo() *Git {
 }
 
 func loadBranchFromEnv() string {
-	varNames := []string{"GIT_BRANCH", "CIRCLE_BRANCH", "TRAVIS_BRANCH", "CI_BRANCH", "APPVEYOR_REPO_BRANCH", "WERCKER_GIT_BRANCH", "BRANCH_NAME"}
+	varNames := []string{"GIT_BRANCH", "CIRCLE_BRANCH", "TRAVIS_BRANCH", "CI_BRANCH", "APPVEYOR_REPO_BRANCH", "WERCKER_GIT_BRANCH", "DRONE_BRANCH", "BRANCH_NAME"}
 	for _, varName := range varNames {
 		if branch := os.Getenv(varName); branch != "" {
 			return branch

gitinfo_test.go

@@ -20,6 +20,7 @@ func TestLoadBranchFromEnv(t *testing.T) {
 				"CI_BRANCH":            "ci-master",
 				"APPVEYOR_REPO_BRANCH": "appveyor-master",
 				"WERCKER_GIT_BRANCH":   "wercker-master",
+				"DRONE_BRANCH":         "drone-master",
 				"BRANCH_NAME":          "jenkins-master",
 			},
 			"master",
@@ -32,6 +33,7 @@ func TestLoadBranchFromEnv(t *testing.T) {
 				"CI_BRANCH":            "ci-master",
 				"APPVEYOR_REPO_BRANCH": "appveyor-master",
 				"WERCKER_GIT_BRANCH":   "wercker-master",
+				"DRONE_BRANCH":         "drone-master",
 				"BRANCH_NAME":          "jenkins-master",
 			},
 			"circle-master",
@@ -43,6 +45,7 @@ func TestLoadBranchFromEnv(t *testing.T) {
 				"CI_BRANCH":            "ci-master",
 				"APPVEYOR_REPO_BRANCH": "appveyor-master",
 				"WERCKER_GIT_BRANCH":   "wercker-master",
+				"DRONE_BRANCH":         "drone-master",
 				"BRANCH_NAME":          "jenkins-master",
 			},
 			"travis-master",
@@ -75,6 +78,13 @@ func TestLoadBranchFromEnv(t *testing.T) {
 			},
 			"jenkins-master",
 		},
+		{
+			"only DRONE_BRANCH defined",
+			map[string]string{
+				"DRONE_BRANCH": "drone-master",
+			},
+			"drone-master",
+		},
 		{
 			"no branch var defined",
 			map[string]string{},
@@ -91,7 +101,7 @@ func TestLoadBranchFromEnv(t *testing.T) {
 }
 
 func resetBranchEnvs(values map[string]string) {
-	for _, envVar := range []string{"CI_BRANCH", "CIRCLE_BRANCH", "GIT_BRANCH", "TRAVIS_BRANCH", "APPVEYOR_REPO_BRANCH", "WERCKER_GIT_BRANCH", "BRANCH_NAME"} {
+	for _, envVar := range []string{"CI_BRANCH", "CIRCLE_BRANCH", "GIT_BRANCH", "TRAVIS_BRANCH", "APPVEYOR_REPO_BRANCH", "WERCKER_GIT_BRANCH", "DRONE_BRANCH", "BRANCH_NAME"} {
 		os.Unsetenv(envVar)
 	}
 	for k, v := range values {

goveralls.go

@@ -253,6 +253,8 @@ func process() error {
 		jobId = semaphoreJobId
 	} else if jenkinsJobId := os.Getenv("BUILD_NUMBER"); jenkinsJobId != "" {
 		jobId = jenkinsJobId
+	} else if droneBuildNumber := os.Getenv("DRONE_BUILD_NUMBER"); droneBuildNumber != "" {
+		jobId = droneBuildNumber
 	}
 
 	if *repotoken == "" {
@@ -271,6 +273,8 @@ func process() error {
 		pullRequest = prNumber
 	} else if prNumber := os.Getenv("PULL_REQUEST_NUMBER"); prNumber != "" {
 		pullRequest = prNumber
+	} else if prNumber := os.Getenv("DRONE_PULL_REQUEST"); prNumber != "" {
+		pullRequest = prNumber
 	}
 
 	sourceFiles, err := getCoverage()