Update zoneinfo files and tzfile.h to 2018c-20-ga6cc661.

For the time being we comment out the tests for pre-1913
offsets in America/Jamaica.

Merge pull request google#51 from mwinterb/vs2017-codegen-fix

Declaring kDigits static to avoid a codegen bug in VS2017.

Merge pull request google#21 from google/v2

Merging v2 into master

Added link to the mailing list.