Update version

ChangeLog.rst

-5.1.0 - TBD
------------
+5.1.0 - 2018-07-05
+------------------
 
 * Added experimental support for RGB color output enabled with
   the ``FMT_EXTENDED_COLORS`` macro:

doc/build.py

@@ -6,7 +6,7 @@ import errno, os, shutil, sys, tempfile
 from subprocess import check_call, check_output, CalledProcessError, Popen, PIPE
 from distutils.version import LooseVersion
 
-versions = ['1.0.0', '1.1.0', '2.0.0', '3.0.2', '4.0.0', '4.1.0', '5.0.0']
+versions = ['1.0.0', '1.1.0', '2.0.0', '3.0.2', '4.0.0', '4.1.0', '5.0.0', '5.1.0']
 
 def pip_install(package, commit=None, **kwargs):
   "Install package using pip."