Contint with gcc8/clang6 and Ubuntu Bionic LTS (#236)

* Updated container to bionic / added support for gcc-8.
* Specify compiler for gcc-6
* Added support for clang 6.

.travis.yml

@@ -37,6 +37,7 @@ matrix:
       - CXXFLAGS=-std=c++11
 
   - os: linux
+    compiler: gcc
     env:
       - COMPILER_PACKAGE=g++-6
       - C_COMPILER=gcc-6
@@ -49,6 +50,13 @@ matrix:
       - C_COMPILER=gcc-7
       - CXX_COMPILER=g++-7
       
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER_PACKAGE=g++-8
+      - C_COMPILER=gcc-8
+      - CXX_COMPILER=g++-8
+
   ################
   # Linux / Clang
   ################
@@ -65,13 +73,19 @@ matrix:
       - C_COMPILER=clang-5.0
       - CXX_COMPILER=clang++-5.0
       
+  - os: linux
+    env:
+      - COMPILER_PACKAGE=clang-6.0
+      - C_COMPILER=clang-6.0
+      - CXX_COMPILER=clang++-6.0
+
 before_install:
 
   # Show OS/compiler version (this may not be the same OS as the Docker container)
   - uname -a
 
   # Use an artful container - gives us access to latest compilers.
-  - docker run -d --name ubuntu-test-container -v $(pwd):/travis ubuntu:artful tail -f /dev/null
+  - docker run -d --name ubuntu-test-container -v $(pwd):/travis ubuntu:bionic tail -f /dev/null
   - docker ps