Integrate Python-based tests and benchmark into Travis

* Unfortunately, due to the SWIG dependency, we need sudo to install on
  Travis (conflicts when adding with debian-sid source prevent using addons)
  which means we cannot use the container-based infrastructure anymore.
* Building the Python bindings requires g++5 (at least with -Werr set).
* When building the Python bindings on Travis, the numpy includes are not found
  by cmake, so they have to be added separately by running a Python process with
  `numpy.get_include()`
* The build script now (somewhat clumsily) depends on the RUN_PYTEST environment
  variable. If it is set to anything other than "", it will make the tests and
  run tests and benchmarks

build.sh

@@ -35,9 +35,20 @@
 # Exit on any error.
 set -e
 
-# Build EKF shared library.
-mkdir Build -p
-cd Build
-cmake ../EKF
-make
-cd ..
+if [ -z ${RUN_PYTEST} ];
+then
+    # Build EKF shared library.
+    mkdir Build -p
+    cd Build
+    cmake ../EKF
+    make
+    cd ..
+else
+    # Build EKF shared library.
+    mkdir Build -p
+    cd Build
+    cmake -DPythonTests=1 ../EKF
+    make pytest
+    make pytest-benchmark
+    cd ..
+fi