Update dnscrypt-resolvers.csv

According to the OpenNIC wiki [1], the correct IPv6 address for PhilipSoutham's server is ns18.ca.us, not ns17.ca.us. The provider key was also wrong.

[1] http://wiki.opennicproject.org/Tier2
Merge pull request #150 from rob/patch-1

Update dnscrypt-resolvers.csv