Fix get_ulid_time in early Python 2.7 versions

[larsyencken]

Adds the b'' string prefix, since unicode struct specifications weren't supported in early minor releases of Python 2.7.

In Python 2.7.6 the unit test was failing for get_ulid_time(). Now they pass again. I also confirmed that they continue to pass for 2.7.13 and 3.6.0.

[larsyencken]

Fixes #2

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master      #3   +/-   ##
======================================
  Coverage    97.7%   97.7%           
======================================
  Files           2       2           
  Lines          87      87           
======================================
  Hits           85      85           
  Misses          2       2

Impacted Files	Coverage Δ
ulid2.py	96.66% <100%> (ø)	✅

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[akx]

Thanks a lot, @larsyencken! Released in ulid2 0.1.1.