Introduce sprig

[yorinasub17]

This integrates sprig into boilerplate so that the sprig functions are available in the templates rendered by `boilerplate.

Note that this isn't a straightforward integration. There are many functions that boilerplate provides that overlap with sprig that have conflicting behavior. To address these, I created an alias for the boilerplate and sprig versions so that both are available, with notes on which ones are supported and which ones are deprecated. See the README update for more details.

This PR also has a few miscellaneous improvements:

• Switch from glide to dep for golang dependency management.
• Update circleci config to use later versions for many things.
• Use gruntwork-cli help text formatters to make it more consistent with our other CLI tools.
• Run gofmt on the files I touched (automatically done by editor).

[yorinasub17]

Going to merge and release this!