Dockerfile.build-env: install git to enable travis builds

The LwIP build requires git to work, so this must be available
in the build-env docker image.

build-env/Dockerfile.build-env

@@ -2,5 +2,5 @@ FROM debian:testing-slim
 
 RUN apt-get update && apt-get install -y autoconf automake gcc clang \
   libtool libtool-bin make pkg-config libcunit1-dev libssl-dev \
-  exuberant-ctags
+  exuberant-ctags git
 RUN apt-get clean