Build flex with current version

.travis.yml

@@ -5,7 +5,7 @@ install:
   - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
   - sudo apt-get -qq update
   - sudo apt-get install -y g++-6 libstdc++-6-dev
-  - sudo apt-get install -y flex valgrind
+  - sudo apt-get install -y valgrind
   - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 90
   - sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 90
 
@@ -16,6 +16,13 @@ install:
   - ./configure && make && sudo make install
   - cd ..
 
+  # Install flex 2.6.4
+  - wget https://github.com/westes/flex/releases/download/v2.6.4/flex-2.6.4.tar.gz
+  - tar -xvzf flex-2.6.4.tar.gz
+  - cd flex-2.6.4
+  - ./configure && make && sudo make install
+  - cd ..
+
   # Show installed versions.
   - which g++
   - g++ -v
@@ -29,9 +36,14 @@ compiler:
   - clang
 
 script:
+  # bi build with flex/bison files checked into repo
+  - make -j4
+  - make test
+  - make test_example
+
+  # build flex/bison files in CI
   - make cleanall
   - make -j4
-
   - make test
   - make test_example