mbedtls: enable Travis CI tests

script/install-deps-linux.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+set -x
+
+if [ "$MBEDTLS" ]; then
+  git clone https://github.com/ARMmbed/mbedtls.git ./deps/mbedtls
+  cd ./deps/mbedtls
+  git checkout mbedtls-2.6.1
+  cmake -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DUSE_SHARED_MBEDTLS_LIBRARY=ON -DUSE_STATIC_MBEDTLS_LIBRARY=OFF .
+  cmake --build .
+
+  echo "mbedTLS built in `pwd`"
+fi