configure.ac: Lower git master version to 1.0.90

The idea was to use 1.1.0 for the git master version, so that git master
would always be newer than any release or 1.1.x version.  This means
we have to skip 1.1 as a release, which turns out to be confusing.

We can just use version 1.0.90 instead, which lets us call the next
release 1.1.0.