Add missed package ca-certificates, automake, libmbedtls-dev.

fix these errors:

- ca-certificates missed
  fatal: unable to access
'https://github.com/shadowsocks/shadowsocks-libev.git/': Problem with
the SSL CA cert (path? access rights?)

- automake missed
  Can't exec "aclocal": No such file or directory at
/usr/share/autoconf/Autom4te/FileUtils.pm line 326.
  autoreconf: failed to run aclocal: No such file or directory
  
- libmbedtls-dev
  checking for mbedtls_cipher_setup in -lmbedcrypto... no
  configure: error: mbed TLS libraries not found.

docker/ubuntu/Dockerfile

@@ -1,7 +1,9 @@
 FROM ubuntu:latest
 MAINTAINER Sah Lee <contact@leesah.name>
 
-ENV DEPENDENCIES git-core gettext build-essential autoconf libtool libssl-dev libpcre3-dev asciidoc xmlto zlib1g-dev libev-dev libudns-dev libsodium-dev
+ENV DEPENDENCIES git-core gettext build-essential autoconf libtool libssl-dev libpcre3-dev \
+                 asciidoc xmlto zlib1g-dev libev-dev libudns-dev libsodium-dev \
+                 ca-certificates automake libmbedtls-dev
 ENV BASEDIR /tmp/shadowsocks-libev
 ENV SERVER_PORT 8338