Version constraint match

[matt-glover]

While looking to build this locally I started by trying to run it with Elixir 1.3 based on the version constraint in mix.exs.

I ran into issues and through trial and error discovered that I need to be on Elixir 1.7+ to build and run all the key tasks for the application. The app appears to build, test, and generally run correctly with Elixir 1.5 with the exception of mix docs.

1.5 throws this error if you try for mix docs so conceivably a library downgrade could be attempted:

** (ExDoc.Retriever.Error) ExDoc 0.19+ requires Elixir v1.7 and later. For earlier Elixir versions, make sure to depend on {:ex_doc, "~> 0.18.0"}

Edit: Based on the Travis CI results it seems like 1.5 support is expected so I selected that version to start with.

Also updated mix.lock because the latest version of hex I pulled down includes the checksums for deps. Happy to move that into a separate PR if that's preferred.

[matt-glover]

Based on the Travis build seems like this needs to be 1.5. Will update.

[bcardarella]

@matt-glover I'll take a look at this in the next few days, DockYard is using this library internally and I'm not sure what version that app is on.