Fix integration tests on CentOS
Fix integration tests on CentOS

This fixes --version integration test on CentOS, as noticed by
https://github.com/projectatomic/skopeo/pull/91 .  The underlying cause
is:
- Makefile builds with -ldflags "-X var=value", while go 1.4.2 only
  supports "-X var value".  This causes CentOS builds to be built
  without the specific commit information
- The --version integration test assumes that commit information will
  always follow the version number.

Changing either one of these would fix the build, changing the
integration test has the advantage that we don't have to use the
obsolete -X syntax and suffer warnings on newer Go versions.

projectatomic/skopeo,jwhonce/skopeo,jwhonce/skopeo,mikebrow/skopeo,mtrmac/skopeo,projectatomic/skopeo,mikebrow/skopeo,mtrmac/skopeo,mtrmac/skopeo