Log Message:
devel/ruby-activesupport51: update to 5.1.6

## Rails 5.1.6 (March 29, 2018) ##

*   Return all mappings for a timezone identifier in `country_zones`

    Some timezones like `Europe/London` have multiple mappings in
    `ActiveSupport::TimeZone::MAPPING` so return all of them instead
    of the first one found by using `Hash#value`. e.g:

        # Before
        ActiveSupport::TimeZone.country_zones("GB") # => \ 
["Edinburgh"]

        # After
        ActiveSupport::TimeZone.country_zones("GB") # => \ 
["Edinburgh", "London"]

    Fixes #31668.

    *Andrew White*