update fastify version to 2

[Eomm]

Closes #11

@allevo Hope is fine for you 👍
I don't know if you would like to update all the dependencies.
This was the output of npm outdated

Package        Current  Wanted  Latest  Location
fastify         1.14.3  1.14.3   2.0.1  fastify-oauth2
nock             9.6.1   9.6.1  10.0.6  fastify-oauth2
simple-oauth2    1.6.0   1.6.0   2.2.1  fastify-oauth2
snazzy           7.1.1   7.1.1   8.0.0  fastify-oauth2
standard        11.0.1  11.0.1  12.0.1  fastify-oauth2

[allevo]

Hi!

thanks!
I'll dump a major version. Maybe 1.x points to fastify 1.x, and 2.x points to fastify 2.x