Make `rimraf` and `gulp-symdest` dependencies instead of devDependencies

[mathphreak]

Currently, rimraf and gulp-symdest are devDependencies in package.json. This means that they will not be installed by default when the package is installed as a dependency of another package. Since index.js require()s them, gulp-atom-electron crashes as soon as it is require()d.

[joaomoreno]

Many thanks!