Explicit esprima@4.0.0 dependency

[gramstrong]

I added an explicit dependency for esprima@4.0.0 in the package.json. Previously a clean install of pretty-swag was using esprima@4.0.0 via a dependency in js-yaml, itself a dependency of json-schema-ref-parser.

This was causing dependency conflicts with other installed modules that require an older version of esprima that isn't compatible with @4.0.0.

[twskj]

Thank you! I totally overlooked it.