Support grunt 1.6

[bbaker2]

grunt-ant-sfdc does not work with the current version of nodejs/grunt

If you upgrade these libraries, you will get

Warning: spawn EINVAL Use --force to continue.

A change in Node.JS prevents grunt.util.spawn from calling ant directly.

I did a little testing and was able to git it working by adding

opts : {
    shell: true
}

to ant_sfdc.js