Bump version.mk.

Naive builders, particular on Windows without git installed,
would get builds calling themselves 1.0.1 even though master
has diverged significantly from the 1.0.x series at this point.

We should update this file both before and after release.

version.mk

-# static version string; update manually every release.
-OPUS_VERSION = "1.0.1"
+# static version string; update manually before and afer every release.
+OPUS_VERSION = "1.1.x"