build: add a check for mercurial/hg

When running `make vendor` without `hg` in the path, the build fails
with the following error:

   [WARN] Unable to checkout bitbucket.org/ww/goautoneg
   [ERROR] Update failed for bitbucket.org/ww/goautoneg: hg is not installed
   [ERROR] Failed to install: hg is not installed
   make: *** [Makefile:74: vendor] Error 1

Glide needs to fetch a repository from bitbucket.org, which is a
Mercurial hosting site. The `hg` executable needs to be available to
download the requires pkgs.

Reported-by: Madhu Rajanna <mrajanna@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>