Add basic Travis-CI support.

- Installs gcc/g++ 6, cmake, and ccache.
- Builds and runs draco_tests.

.travis.yml

@@ -0,0 +1,29 @@
+dist: trusty
+sudo: require
+language: cpp
+compiler: gcc
+
+addons:
+  apt:
+    sources:
+    - ubuntu-toolchain-r-test
+    packages:
+    - gcc-6
+    - g++-6
+    - ccache
+    - cmake
+
+script:
+  # Link gcc-6 and g++-6 to their standard commands
+  - ln -s /usr/bin/gcc-6 /usr/local/bin/gcc
+  - ln -s /usr/bin/g++-6 /usr/local/bin/g++
+  # Export CC and CXX to tell cmake which compiler to use
+  - export CC=/usr/bin/gcc-6
+  - export CXX=/usr/bin/g++-6
+  # Check versions of gcc, g++ and cmake
+  - gcc -v && g++ -v && cmake --version
+  # Configure and build
+  - mkdir _travis_build && cd _travis_build
+  - cmake -G "Unix Makefiles" -DENABLE_TESTING=ON -DENABLE_CCACHE=ON ..
+  - make -j
+  - ./draco_tests