Release Spot 2.6.1

* NEWS, configure.ac, doc/org/setup.org: Update version number.

NEWS

-New in spot 2.6.0.dev  (not yet released)
+New in spot 2.6.1  (2018-08-04)
 
   Command-line tools:
 
@@ -13,13 +13,13 @@ New in spot 2.6.0.dev  (not yet released)
   - The spot::tl_simplifier class could raise an exception while
     attempting to reduce formulas containing unsimplified <->, -> or
     xor, if options nenoform_stop_on_boolean and synt_impl are both
-    set.  (This combinations of options is not available from
+    set.  (This combination of options is not available from
     command-line tools.)
 
   - The spot::contains(a, b) function introduced in 2.6 was testing
     a⊆b instead of a⊇b as one would expect.  Infortunately the
     documentation was also matching the code, so this is a backward
-    incompatible change.
+    incompatible change, but a short-lived one.
 
   - The Python binding of the getter of spot::parsed_formula::f was
     returning a reference instead of a copy, causing issues if the

configure.ac

@@ -21,7 +21,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 AC_PREREQ([2.61])
-AC_INIT([spot], [2.6.0.dev], [spot@lrde.epita.fr])
+AC_INIT([spot], [2.6.1], [spot@lrde.epita.fr])
 AC_CONFIG_AUX_DIR([tools])
 AC_CONFIG_MACRO_DIR([m4])
 AM_INIT_AUTOMAKE([1.11 gnu tar-ustar color-tests parallel-tests])

doc/org/setup.org

 #+OPTIONS: H:2 num:nil toc:t html-postamble:nil
 #+EMAIL: spot@lrde.epita.fr
 #+HTML_LINK_HOME: index.html
-#+MACRO: SPOTVERSION 2.6
-#+MACRO: LASTRELEASE 2.6
-#+MACRO: LASTTARBALL [[http://www.lrde.epita.fr/dload/spot/spot-2.6.tar.gz][=spot-2.6.tar.gz=]]
-#+MACRO: LASTNEWS [[https://gitlab.lrde.epita.fr/spot/spot/blob/spot-2-6/NEWS][summary of the changes]]
+#+MACRO: SPOTVERSION 2.6.1
+#+MACRO: LASTRELEASE 2.6.1
+#+MACRO: LASTTARBALL [[http://www.lrde.epita.fr/dload/spot/spot-2.6.1.tar.gz][=spot-2.6.1.tar.gz=]]
+#+MACRO: LASTNEWS [[https://gitlab.lrde.epita.fr/spot/spot/blob/spot-2-6-1/NEWS][summary of the changes]]
 #+MACRO: LASTDATE 2018-07-04
 
 #+ATTR_HTML: :id spotlogo