Remove conditional checks for Process.clock_gettime

[marcotc]

Process.clock_gettime is now available for all supported versions of Ruby.
This PR removes conditional checks for its presence.

Ruby 2.0 deprecation checklist

• Remove 2.0 from CI
• Remove 2.0-specific code paths
• Update to 2.1+ standards: https://github.com/ruby/ruby/blob/v2_1_0/NEWS
  • Module#include and Module#prepend are now public methods.
  • Exception#cause (no changes)
  • Array#to_h converts an array of key-value pairs into a Hash.
  • Kernel#singleton_method (no changes)
  • Process.clock_gettime
  • Enumerable#to_h converts a list of key-value pairs into a Hash.
  • Now the default values of keyword arguments can be omitted. Those “required keyword arguments” need giving explicitly at the call time.
• Update documentation

[ivoanjo]

Thank you so much, kind sir! :)