Update jwt library

[sideshow]

• Resolves Replace github.com/dgrijalva/jwt-go with github.com/golang-jwt/jwt #186
• Resolves chore: stop using vulnerable, unmaintained jwt library (resolves #186) #187
• Resolves Replace github.com/dgrijalva/jwt-go #189

[coveralls]

Coverage remained the same at 100.0% when pulling d6a92e9 on chore/update-jwt-library into 33f1b34 on master.