msgmerge: avoid including native extensions in gettext gem

MsgMerge uses Text::Levenshtein instead of ::Levenshtein.

Benchmark:
  It used the average size of msgids in rroonga's PO file as the size
  of each String object computed a Levenshtein distance.

  % ruby levenshtein-benchmark.rb
  average: 72.81869918699186
                     user     system      total        real
  Levenshtein:   0.000000   0.000000   0.000000 (  0.003926)
  Text:          0.000000   0.000000   0.000000 (  0.002784)

  % ruby -e "puts 0.002784 / 0.003926" #=> 0.7091186958736628