Allow newer jwt versions (>=2.1.0)

[gencer]

Due to strict versioning, jwt may clash with existing other gems. To fix this, we allow JWT lib equal and greater then 1.5.2

See: #23 and #21

[coveralls]

Coverage decreased (-12.2%) to 84.706% when pulling c7df4ec on gencer:patch-1 into cbc68b5 on chriswarren:master.

[gencer]

Hm.. I'm thinking this failures could be the reason of Ruby v2.2.1 targeted. Will test more then update PR.

[chriswarren]

It looks like 11 of the 12 failures are around 2 identical problems, so getting tests passing hopefully won't be too bad.

I won't be able to merge this until you've addressed them. I think doing that should raise the test coverage back up as well.

If you're not interested in doing that we can close this until I have time to look at it, but I don't know when that will be.

[nbulaj]

Closing in favor of #27