Fix in style/babel.el.

* style/babel.el (LaTeX-babel-active-languages): Move `nreverse'
call inside `let'.
[Fix #19281]  Allow moodifying environments with macros in their names.

* latex.el (LaTeX-environment-name-regexp): New function.
(LaTeX-modify-environment): Use it.
(LaTeX-find-matching-end): Use `TeX-grop' instead of the
hard-coded "{".  Search for the closing of the environment with
`re-search-forward' instead of `search-forward' and use
`LaTeX-environment-name-regexp': this allows to consider
environments with macros in their names.  (Bug#19281)
(LaTeX-find-matching-begin): Use `TeX-grop' instead of the
hard-coded "{".