Update dependencies, fix compiler-breaking typo

[lilybarrett]

We initially ran into issues incorporating Laboratory in an application that depends on a later version of cowboy. This PR does the following:

• Removes direct invocation of cowboy in favor of plug-cowboy
• Updates to a more modern Elixir version
• Minimally adds a few more modern touches to other code
• Fixes a typo in mix.exs that broke the compiler (referencing package instead of package().)

Also worth noting: We currently have different sources of truth for the Laboratory library across different MBTA applications. For instance, AlertsViewer uses a branch of Paul's personal fork, and DotCom uses a branch of my personal fork. The MBTA's fork is currently very out of date compared to both of these!

Hopefully, with the PR changes, the MBTA's fork can become the sole repo used for Laboratory.

[lilybarrett]

@arkadyan @paulswartz I closed the original PR to Paul's fork, and opened this one so that the changes will be reflected in the MBTA's Laboratory fork.

[arkadyan]

👍

[arkadyan]

@lilybarrett With this change, can we migrate dotcom to use this library source?

[lilybarrett]

@lilybarrett With this change, can we migrate dotcom to use this library source?

Yes, definitely! I plan to make another PR to update this in Dotcom once we merge this.

UPDATED: PR for Dotcom is here: mbta/dotcom#3278