Changed language in notification section of settings from 'select ringtone' to 'sound'.
Switched string constant name for notification sound from preferences__select_reingtone to preferences_sound to match the new value of the constant, 'Sound'.
Merge pull request #358 from WhisperSystems/issue353

"Select Ringtone" preference in "Notifications" section of TextSecure settings now reads "Sound" to better reflect the selection of audio presented.