gulpfile.js: Use "run-sequence" to run default tasks

Instead of specifying tasks that are part of the "default" task using
dependencies and running them in parallel, execute them serially using
"run-sequence". This makes things deterministic and ensures that all
messages from all tasks are seen in the output (which wasn't the case
before, leading to confusion).

gulpfile.js

@@ -13,6 +13,7 @@ let jasmine = require("gulp-jasmine");
 let package_ = require("./package");
 let peg = require("./lib/peg");
 let rename = require("gulp-rename");
+let runSequence = require("run-sequence");
 let source = require("vinyl-source-stream");
 let spawn = require("child_process").spawn;
 let transform = require("gulp-transform");
@@ -102,4 +103,6 @@ gulp.task("parser", () =>
 );
 
 // Default task.
-gulp.task("default", ["lint", "spec"]);
+gulp.task("default", cb =>
+  runSequence("lint", "spec", cb)
+);

package.json

@@ -59,6 +59,7 @@
     "gulp-transform": "1.0.8",
     "gulp-uglify": "2.0.0",
     "morgan": "1.7.0",
+    "run-sequence": "^1.2.2",
     "vinyl-buffer": "1.0.0",
     "vinyl-source-stream": "1.1.0"
   },