Remove boot_fit_nb()

[billdenney]

Related to #51, remove extraneous function switching from boot_fit_nb() to boot_fit() to ensure fix for update() works throughout.

[billdenney]

FYI, the reason that boot_fit_nb() is now extraneous is that in some previous version there was a switch from using glm.nb() to update(). Now that the glm.nb method is using update(), it should work the same way for both glm and glm.nb models.