dtrace-provider dependency inadvertently removed by #101

[melloc]

In #101 (acadfc4c), the "dtrace-provider" dependency was moved into an "optionalDependency" field. The correct name for the field is "optionalDependencies". I'll submit a PR against master, but I'd also like to get this backported to a 1.5.3 release if possible.

[trentm]

• published restify-clients@2.0.2 with this fix
• created a 1.x maint branch off of commit bbdeb79 (as suggested in #169)
• published restify-clients@1.5.3 with this fix as well

$ npm info restify-clients dist-tags
{ latest: '2.0.2', 'latest-1': '1.5.3' }