Replaced #next_vowel_index with a version that is slightly more performant and isn't affected by https://github.com/rubinius/rubinius/issues/2625.
Replaced #next_vowel_index with a version that is slightly more performant and isn't affected by https://github.com/rubinius/rubinius/issues/2625.

josephwilk/pronounce,josephwilk/pronounce,gwagener/pronounce,josephwilk/pronounce,gwagener/pronounce,gwagener/pronounce