corrected test for gpg extension in hg

At least in mercurial 2.4.2 to check if gpg extension is installed one should do `hg showconfig extensions.hgext.gpg` and if it returns anything, but empty string, the extension is enabled. Even "\n" indicates that extension is turned on, so chomp is invalid.
Merge pull request #9 from wrzasa/patch-1

corrected test for gpg extension in hg