Do not check version of docker-squash

It's responsibility of user to provide working tools to run
build+test (mentioned in README).

The docker-squash==1.0.5 version is quite old and requires
docker-py module.  Newer python docker related tools use docker
module, which conflicts with docker-py.  So if some new tool is
installed in same environment (for example conu framework) its
requirement docker module overwrites docker-py.

Follow up for 0f93964 and 55d1108.

Fixes: sclorg#100

README.md

@@ -93,7 +93,7 @@ Just run `make check`
 Dependencies for testsuite:
 
 - docker
-- docker-squash (version 1.0.5)
+- docker-squash
 - git
 - go-md2man
 - make

build.sh

@@ -129,11 +129,7 @@ squash ()
   local base squashed_from squashed= unsquashed=$IMAGE_ID
   test "$SKIP_SQUASH" = 1 && return 0
 
-  # FIXME: We have to use the exact versions here to avoid Docker client
-  #        compatibility issues
-  local squash_version=1.0.5
-  test "$(docker-squash --version 2>&1)" = "$squash_version" || \
-      error "docker-squash $squash_version required"
+  docker-squash --help &>/dev/null || error "docker-squash is required"
 
   if test -f .image-id.squashed; then
       squashed=$(cat .image-id.squashed)