Use the container based travis builds

Travis has upgraded to use docker images on EC2. This allows much faster
build times. This patch forces the build to use the containers.

.travis.yml

@@ -1,8 +1,6 @@
 language: c
 
-# This should be false to support container based building but the apt addons
-# don't seem to work right now
-sudo: required
+sudo: false
 
 compiler:
   - gcc
@@ -52,6 +50,7 @@ before_script:
   - cmake --version
   - cmake
     -G "Unix Makefiles"
+    "-DCIVETWEB_THIRD_PARTY_DIR=${HOME}/third-party"
     -DCMAKE_BUILD_TYPE=${BUILD_TYPE}
     -DBUILD_SHARED_LIBS=${BUILD_SHARED}
     -DCIVETWEB_ENABLE_SSL=${ENABLE_SSL}
@@ -65,6 +64,10 @@ before_script:
     -DCIVETWEB_CXX_STANDARD=${CXX_STANDARD}
     ..
 
+cache:
+  directories:
+  - $HOME/third-party
+
 matrix:
   - fast_finish: true