Add Buildkite support

ci_info.js

@@ -60,6 +60,14 @@ module.exports = {
         commit_sha:       env.APPVEYOR_REPO_COMMIT,
         pull_request:     env.APPVEYOR_PULL_REQUEST_NUMBER,
       }
+    } else if (env.BUILDKITE) {
+      return {
+        name:             "buildkite",
+        build_identifier: env.BUILDKITE_BUILD_ID,
+        build_url:        env.BUILDKITE_BUILD_URL,
+        branch:           env.BUILDKITE_BRANCH,
+        commit_sha:       env.BUILDKITE_COMMIT
+      }
     } else {
       return {};
     }

test/test.js

@@ -48,5 +48,12 @@ describe('ci_info', function() {
       assert.equal(ci.name, 'appveyor');
     });
 
+    it('should return buildkite as name if process.env.BUILDKITE is set', function() {
+      process.env.BUILDKITE = 'true';
+
+      var ci = CiInfo.getInfo();
+      assert.equal(ci.name, 'buildkite');
+    });
+
   });
 });