version: only append "-pre" to version if no tag
I'm using the TAG
for nightlies (not using the NIGHTLY
mechanism in the Makefile exactly as designed, but that's a future pull request) and would like to not have -pre
on the end for them. With this change, nightlies come out like this:
$ ./iojs -v
v1.0.0-nightly20150110a32b92dbcf