fix: update root url to compute.googleapis.com

[JustinBeckwith]

This change will update the root url used for requests from www.googleapis.com to compute.googleapis.com.

[bcoe]

tests are failing due to a few assertions that don't take into account compute:

      AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
+ expected - actual

- 'https://www.googleapis.com/compute/v1/zones/us-central1-a/machineTypes/n1-standard-2'
+ 'https://compute.googleapis.com/compute/v1/zones/us-central1-a/machineTypes/n1-standard-2'
      + expected - actual

[codecov]

Codecov Report

Merging #349 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #349   +/-   ##
=======================================
  Coverage   99.14%   99.14%           
=======================================
  Files          20       20           
  Lines        1524     1524           
=======================================
  Hits         1511     1511           
  Misses         13       13

Impacted Files	Coverage Δ
src/service.js	100% <100%> (ø)	⬆️
src/vm.js	97.48% <100%> (ø)	⬆️
src/index.js	100% <100%> (ø)	⬆️

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[bcoe]

this is just for the benefit of a public API, so that the user can view the url of the resource that the VM is based on:

    it('should localize the URL of the VM', function() {
      assert.strictEqual(
        vm.url,
        [
          'https://www.googleapis.com/compute/v1/projects',
          COMPUTE.projectId,
          'zones',
          ZONE.name,
          'instances',
          VM_NAME,
        ].join('/')
      );
    });