update cygwin installation docs

cython is installed with pip later.
liblz4 is available from normal cygwin packages now.
creation of virtualenv worked now without issues.

docs/installation.rst

@@ -187,15 +187,14 @@ Cygwin
 
 .. note::
     Running under Cygwin is experimental and has only been tested with Cygwin
-    (x86-64) v2.1.0.
+    (x86-64) v2.5.2.
 
 Use the Cygwin installer to install the dependencies::
 
     python3 python3-setuptools
-    python3-cython  # not needed for releases
     binutils gcc-g++
     libopenssl openssl-devel
-    liblz4_1 liblz4-devel  # from cygwinports.org
+    liblz4_1 liblz4-devel
     git make openssh
 
 You can then install ``pip`` and ``virtualenv``::
@@ -203,10 +202,6 @@ You can then install ``pip`` and ``virtualenv``::
     easy_install-3.4 pip
     pip install virtualenv
 
-In case the creation of the virtual environment fails, try deleting this file::
-
-    /usr/lib/python3.4/__pycache__/platform.cpython-34.pyc
-
 
 .. _pip-installation: