Fix latex-mode expansions setup bug

The hook for latex-mode is LaTeX-mode-hook and the major mode for it is
latex-mode. So the function `er/enable-mode-expansions' that assumes
that the former can be inferred from the latter fails. A solution is to
temporarily bind `latex-mode-hook' to `Latex-mode-hook' and save any
modification after the call.
Merge pull request #119 from thisirs/master

Fix latex-mode expansions setup bug