This avoids confusion with the meaning of "release" used by
AX_IS_RELEASE. AX_IS_RELEASE is about facts about the source tree,
namely the distinction between releases (tags) and random snapshots.
The build variants in .travis.yml are about facts about the build
being done, namely the distinction between production and
debug/developer builds.

Production builds are sometimes referred to as "release builds",
for example in typical CMake and MSVC build environments, but a
different term seems better here.

Signed-off-by: Simon McVittie <smcv@debian.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=97357
[smcv: cherry-picked from master to dbus-1.10 to get the Travis-CI setup
consistent between the two branches]