gulpfile: Remove reference to missing test task

[andersk]

Description

The task was removed in 4d60ccc, causing this error:

$ npx gulp
[17:21:24] Using gulpfile ~/js/stacktrace.js/gulpfile.js
[17:21:24] Starting 'clean'...
[17:21:24] Finished 'clean' after 4.59 ms
[17:21:24] Starting 'default'...
[17:21:24] 'default' errored after 436 μs
[17:21:24] Error: Task test is not configured as a task on gulp.  If this is a submodule, you may need to use require('run-sequence').use(gulp).
    at /home/anders/js/stacktrace.js/node_modules/run-sequence/index.js:20:10
    at Array.forEach (<anonymous>)
    at verifyTaskSets (/home/anders/js/stacktrace.js/node_modules/run-sequence/index.js:13:11)
    at runSequence (/home/anders/js/stacktrace.js/node_modules/run-sequence/index.js:92:2)
    at Gulp.<anonymous> (/home/anders/js/stacktrace.js/gulpfile.js:53:5)
    at module.exports (/home/anders/js/stacktrace.js/node_modules/orchestrator/lib/runTask.js:34:7)
    at Gulp.Orchestrator._runTask (/home/anders/js/stacktrace.js/node_modules/orchestrator/index.js:273:3)
    at Gulp.Orchestrator._runStep (/home/anders/js/stacktrace.js/node_modules/orchestrator/index.js:214:10)
    at /home/anders/js/stacktrace.js/node_modules/orchestrator/index.js:279:18
    at finish (/home/anders/js/stacktrace.js/node_modules/orchestrator/lib/runTask.js:21:8)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• node_modules/.bin/jscs -c .jscsrc stacktrace.js passes without errors
  • You don’t use JSCS anymore.
• npm test passes without errors
• I have read the contribution guidelines
• I have updated the documentation accordingly
• I have added tests to cover my changes