pre-commit hook to re-write the TravisCI Badge with the current branch. Save as `.git/hooks/pre-commit` (without the .py extension)

pre-commit.py

#!/usr/bin/python
"""
Referencing current branch in github readme.md[1]

This pre-commit hook[2] updates the README.md file's
Travis badge with the current branch

[1] http://stackoverflow.com/questions/18673694/referencing-current-branch-in-github-readme-md
[2] http://www.git-scm.com/book/en/v2/Customizing-Git-Git-Hooks
[3] https://docs.travis-ci.com/user/status-images/
"""
import subprocess

# Hard-Coded for your repo (ToDo: get from remote?)
GITHUB_USER="joegattnet"
REPO="joegattnet_v3"

print "Starting pre-commit hook..."

BRANCH=subprocess.check_output(["git",
                                "rev-parse",
                                "--abbrev-ref",
                                "HEAD"]).strip()

# String with hard-coded values
# See Embedding Status Images[3] for alternate formats (private repos, svg, etc)

#  [![Build Status](https://travis-ci.org/
#  joegattnet/joegattnet_v3.png?
#  branch=staging)][travis]

# Output String with Variable substitution
travis="[![Build Status](https://travis-ci.org/" \
       "{GITHUB_USER}/{REPO}.png?" \
       "branch={BRANCH})][travis]\n".format(BRANCH=BRANCH,
                                          GITHUB_USER=GITHUB_USER,
                                          REPO=REPO)

sentinal_str="[![Build Status]"

readmelines=open("README.md").readlines()
with open("README.md", "w") as fh:
    for aline in readmelines:
        if sentinal_str in aline and travis != aline:
            print "Replacing:\n\t{aline}\nwith:\n\t{travis}".format(
                   aline=aline,
                   travis=travis)
            fh.write(travis)
        else:
            fh.write(aline)

subprocess.check_output(["git", "add", "README.md" ])

print "pre-commit hook complete."

I updated your work here to handle much more case and support Python 3.

I also added a license so authors are protected. Please tell me if GPL v3 doesn't suit you.

https://gist.github.com/jclaveau/af2271b9fdf05f7f1983f492af5592f8