Require Ruby 2.4

[dazuma]

Update Ruby dependency to 2.4, and update documentation to match.

Also updates:

• googleauth dependency to ~> 0.10.0 which will be the googleauth with the same Ruby 2.4 requirement.
• grpc and protobuf dependencies to the latest.
• googleapis-common-protos dependency to the latest.

Note: Tests will fail until googleauth 0.10.0 is actually released.

[codecov[bot]]

Codecov Report

Merging #222 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #222   +/-   ##
======================================
  Coverage    98.2%   98.2%           
======================================
  Files          22      22           
  Lines        2287    2287           
======================================
  Hits         2246    2246           
  Misses         41      41

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