Specify MongoDB >= 2.6
Specify MongoDB >= 2.6

Need to use MongoDB >= 2.6 otherwise get wire version errors on start:
```
UnhandledPromiseRejectionWarning: MongoError: Server at 127.0.0.1:27017 reports wire version 0, but this version of Node.js Driver requires at least 2 (MongoDB2.6).
```
wistityhq/strapi,wistityhq/strapi