force all times to UTC

Prior to this commit, the tests were checking that the timestamps
returned for `@user.basics.updated_at` (and the like) were in UTC
`+00:00`; however `Time.at` [1] will, by default, prefer to output the
timestamp in the local timezone. This commit forces all timestamps to
prefer UTC when output.

NOTE: This change is suboptimal and temporary. A far better solution
would for the tests to be robust enough to handle the local timezones.

[1] http://www.ruby-doc.org/core-2.0.0/Time.html#method-i-utc