Update pip to 8.1.0

https://pip.pypa.io/en/stable/news/
https://github.com/pypa/pip/compare/8.0.2...8.1.0

Archive verified via:
https://pypi.python.org/packages/source/p/pip/pip-8.1.0.tar.gz.asc

And Donald's key from:
https://pypi.python.org/security

$ gpg --verify pip-8.1.0.tar.gz.asc pip-8.1.0.tar.gz
gpg: Signature made Sat, Mar  5, 2016  4:57:30 PM GMT using RSA key ID 3372DCFA
gpg: Good signature from "Donald Stufft (dstufft) <donald@stufft.io>"
gpg:                 aka "Donald Stufft (dstufft) <donald@python.org>"
...
Primary key fingerprint: 7C6B 7C5D 5E2B 6356 A926  F04F 6E3C BCE9 3372 DCFA

CHANGELOG.md

@@ -12,6 +12,8 @@ Improved GDAL support.
 - The vendored GDAL library has been updated to 1.11.1.
 - GDAL bootstrapping now also installs the GEOS and Proj.4 libraries.
 
+Updated pip to 8.1.0.
+
 ## v77 (2016-02-10)
 
 Improvements to warnings and minor bugfix.

bin/compile

@@ -34,7 +34,7 @@ LEGACY_TRIGGER="lib/python2.7"
 DEFAULT_PYTHON_VERSION="python-2.7.11"
 DEFAULT_PYTHON_STACK="cedar-14"
 PYTHON_EXE="/app/.heroku/python/bin/python"
-PIP_VERSION="8.0.2"
+PIP_VERSION="8.1.0"
 SETUPTOOLS_VERSION="19.6"
 
 # Common Problem Warnings