debian: update gmime dependencies

- build against gmime 3.0 by preference
- drop gmime-2.4-dev, no longer supported upstream

debian/changelog

@@ -1,3 +1,11 @@
+notmuch (0.25-2) unstable; urgency=medium
+
+  * Drop build-dep on libgmime-2.4-dev, long unsupported upstream
+  * Bug fix: "please transition to gmime 3.0", thanks to Daniel Kahn
+    Gillmor (Closes: #867353).
+
+ -- David Bremner <bremner@debian.org>  Wed, 26 Jul 2017 10:59:14 -0400
+
 notmuch (0.25-1) unstable; urgency=medium
 
   * New upstream release

debian/control

@@ -11,7 +11,7 @@ Build-Depends:
  debhelper (>= 9),
  pkg-config,
  libxapian-dev,
- libgmime-2.6-dev (>= 2.6.7~) | libgmime-2.4-dev,
+ libgmime-3.0-dev | libgmime-2.6-dev (>= 2.6.7~),
  libtalloc-dev,
  libz-dev,
  python-all (>= 2.6.6-3~),