pip3 install failure: missing libxml

[Adam Sah]

Ubuntu 16.04.3

pip3 install ast2vec

In file included from bblfsh/libuast/uast.c:2:0:
bblfsh/libuast/uast_private.h:8:25: fatal error: libxml/tree.h: No such file or directory
compilation terminated.
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

"sudo apt-get install libxml2-dev" appears to fix this.

[Vadim Markovtsev]

Thanks! I will include this into the readme. We've got the complete dependecnies list in .travis.yml