提高代码健壮性

[kitelife]

在某些sentinel或redis服务器挂了之后，redis-sentinel-manager运行可能会遇到问题

[gslsf223]

/opt/redis-sentinel-ui/gulpfile.js:7
const gulp = require('gulp');
^^^^^
SyntaxError: Use of const in strict mode.
at Module._compile (module.js:439:25)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.require (module.js:364:17)
at require (module.js:380:17)
at Liftoff.handleArguments (/opt/redis-sentinel-ui/node_modules/gulp/bin/gulp.js:116:3)
at Liftoff. (/opt/redis-sentinel-ui/node_modules/gulp/node_modules/liftoff/index.js:198:16)
at module.exports (/opt/redis-sentinel-ui/node_modules/gulp/node_modules/liftoff/node_modules/flagged-respawn/index.js:17:3)
at Liftoff. (/opt/redis-sentinel-ui/node_modules/gulp/node_modules/liftoff/index.js:190:9)