Remove loopback-explorer from dev deps

 1. loopback-explorer has a peer dependency on loopback, which
  forces `npm install` to install loopback within loopback. Somehow
  npm ends up installing loopback 1.x, which is not compatible with
  datasource-juggler 2.x.

 2. The only place using loopback-explorer is the app used by e2e tests.
  However, the e2e test app does not load the explorer at the moment
  and the e2e test are not being run anyway.
Merge pull request #328 from strongloop/remove-explorer-from-deps

[2.0] Remove loopback-explorer from dev deps