Fix DISPLAY detection when LANG is set to a non-English locale.

Force the vim client process to execute with LC_ALL and LANGUAGE set to
en_US.UTF-8, which ensures that the "No display" message returned by the
client can be recognised directly.

References #25.
Merge pull request #26 from malcolmr/fix-non-en-display-check

Fix DISPLAY detection when LANG is set to a non-English locale.