Fix setup.sh on Debian 8

Currently setup.sh keeps failing for me on a Debian 8 VM. Turns out the
error is similar to here pyca/cryptography#2280, which is due to apt
installed of cffi version being less than 1.1.

Command /usr/bin/python -c "import setuptools,
tokenize;__file__='/tmp/pip_build_root/cryptography/setup.py';exec(compile(getattr(tokenize,
'open', open)(__file__).read().replace('\r\n', '\n'), __file__,
'exec'))" install --record /tmp/pip-X8h31p-record/install-record.txt
--single-version-externally-managed --compile failed with error code 1
in /tmp/pip_build_root/cryptography Storing debug log for failure in
/home/nikolai/.pip/pip.log

This PR also fixes #261 #267 #268

setup.sh script now installs libssl-dev #261
added cffi to requirements.txt file which fixes the error in pyca/cryptography#2280 and #268

requirements.txt

@@ -66,3 +66,4 @@ git+https://github.com/mjs/imapclient.git@77047bafd9a82f3bb1faa5e909d776b7a2e1b4
 tldextract==1.7.5
 tox==2.3.1
 nylas==1.2.3
+cffi>=1.6

setup.sh

@@ -90,6 +90,7 @@ apt-get -y install git \
                    libxslt-dev \
                    lib32z1-dev \
                    libffi-dev \
+                   libssl-dev \
                    pkg-config \
                    python-lxml \
                    tmux \