Bump gcc from 4.8 to 4.9 as undefined reference

... to `__warn_memset_zero_len'.
Also remove gcc-6, bump gcc-7 to gcc-9, clang from 5.0 to 8.0

.travis.yml

@@ -18,7 +18,7 @@ matrix:
       addons:
         apt:
           packages:
-            - g++-4.8
+            - g++-4.9
             - gperf
             - build-essential
             - bison
@@ -37,35 +37,7 @@ matrix:
             - libboost-python-dev
             - libboost-filesystem-dev
       env:
-        - MATRIX_EVAL="CONFIG=gcc && CC=gcc-4.8 && CXX=g++-4.8"
+        - MATRIX_EVAL="CONFIG=gcc && CC=gcc-4.9 && CXX=g++-4.9"
-
-    # Latest gcc-6 on Travis Linux
-    - os: linux
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - g++-6
-            - gperf
-            - build-essential
-            - bison
-            - flex
-            - libreadline-dev
-            - gawk
-            - tcl-dev
-            - libffi-dev
-            - git
-            - graphviz
-            - xdot
-            - pkg-config
-            - python
-            - python3
-            - libboost-system-dev
-            - libboost-python-dev
-            - libboost-filesystem-dev
-      env:
-        - MATRIX_EVAL="CONFIG=gcc && CC=gcc-6 && CXX=g++-6"
 
     # Latest gcc supported on Travis Linux
     - os: linux
@@ -74,7 +46,7 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
           packages:
-            - g++-7
+            - g++-9
             - gperf
             - build-essential
             - bison
@@ -93,7 +65,7 @@ matrix:
             - libboost-python-dev
             - libboost-filesystem-dev
       env:
-        - MATRIX_EVAL="CONFIG=gcc && CC=gcc-7 && CXX=g++-7"
+        - MATRIX_EVAL="CONFIG=gcc && CC=gcc-9 && CXX=g++-9"
 
     # Clang which ships on Trusty Linux
     - os: linux
@@ -129,9 +101,9 @@ matrix:
       addons:
         apt:
           sources:
-            - llvm-toolchain-trusty-5.0
+            - llvm-toolchain-bionic-8.0
           packages:
-            - clang-5.0
+            - clang-8.0
             - gperf
             - build-essential
             - bison
@@ -150,7 +122,7 @@ matrix:
             - libboost-python-dev
             - libboost-filesystem-dev
       env:
-        - MATRIX_EVAL="CONFIG=clang && CC=clang-5.0 && CXX=clang++-5.0"
+        - MATRIX_EVAL="CONFIG=clang && CC=clang-8.0 && CXX=clang++-8.0"
 
 #   # Latest clang on Mac OS X
 #   - os: osx