Install gdb 8 for CI:

A bug in gdb is crashing when demangling some symbols. Gdb needs to be updated
to at least gdb 8.

Only run the debug build under gdb (other builds can time out).

.travis.yml

@@ -10,6 +10,7 @@ env:
     # namepart must match the folder name internal
     # to boost's .tar.gz.
     - LCOV_ROOT=$HOME/lcov
+    - GDB_ROOT=$HOME/gdb
     - BOOST_ROOT=$HOME/boost_1_60_0
     - BOOST_URL='http://sourceforge.net/projects/boost/files/boost/1.60.0/boost_1_60_0.tar.gz'
 
@@ -27,6 +28,8 @@ addons:
       - binutils-gold
       # Provides a backtrace if the unittests crash
       - gdb
+      # needed to build gdb
+      - texinfo
 
 matrix:
   include: