Fix make rpm for unreleased code

When the topmost commit isn't tagged, rpmbuild otherwise fails with

    error: line 2: Illegal char '-' (0x2d) in: Version: 176-1-g9101a30a

Replace the dashes from `git describe` with periods, to get a valid RPM
upstream version number.

Closes #45

Makefile

@@ -1,6 +1,6 @@
 # extract name from package.json
 PACKAGE_NAME := $(shell awk '/"name":/ {gsub(/[",]/, "", $$2); print $$2}' package.json)
-VERSION := $(shell git describe 2>/dev/null || echo 1)
+VERSION := $(shell T=$$(git describe 2>/dev/null) || T=1; echo $$T | tr '-' '.')
 ifeq ($(TEST_OS),)
 TEST_OS = centos-7
 endif