Check Emacs version at start

I ran into this when I tried to use ghc-mod with Emacs 24.2, and
got a mysterious error telling me that defvar-local was not defined.
Add test for no errors case
Suppress empty line
checking if the buffer exists (#256).
fixing a comment.
making ghc-mod-running a global var (##255).
setting sentinel.
using buffer-live-p (#256).
ver bumps up.
Fix problems with GHC's Typed Holes
ver bumps up.
enabling -fno-warn- (#246).
fix #271
ver bumps up
ghc-debug displays PATH env.
removing a warning of elisp.
updating .travis.yml.

See c5776c220b2cf68a497a7479e112161b21b6da84.
fix #276
ver bumps up.
removing -threaded