travis: Turn off Homebrew updates

The builds no longer succeed on the xcode9.4 image because updating
Homebrew takes too much time. The packages we need from Homebrew are not
frequently updated, so using the latest formulae that the image provides
should be sufficient.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
diff --git a/.travis.yml b/.travis.yml
index 35326e3..e92f0c4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -47,7 +47,6 @@
             - automake
             - libtool
             - m4
-        update: true
 
 before_script:
     - ./bootstrap.sh
diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index 40532d3..e93a2fe 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11588
+#define LIBUSB_NANO 11589