I worked around this issue with a git pre-commit hook that re-writes the Travis line in the README.md with the current branch.  An example of usage and pre-commit (Python) code (for the question as asked) are below.
Usage
dandye$ git checkout -b feature123 origin/master
Branch feature123 set up to track remote branch master from origin.
Switched to a new branch 'feature123'
dandye$ echo "* Feature123" >> README.md 
dandye$ git add README.md 
dandye$ git commit -m "Added Feature123"
Starting pre-commit hook...
Replacing:
    [][travis]
with:
    [][travis]
pre-commit hook complete.
[feature123 54897ee] Added Feature123
 1 file changed, 2 insertions(+), 1 deletion(-)
dandye$ cat README.md |grep "Build Status"
[][travis]
dandye$ 
dandye$ cat .git/hooks/pre-commit
#!/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. Gist at[4].
[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/
[4] https://gist.github.com/dandye/dfe0870a6a1151c89ed9
"""
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)
#  [][travis]
# Output String with Variable substitution
travis="[][travis]\n".format(BRANCH=BRANCH,
                                            GITHUB_USER=GITHUB_USER,
                                            REPO=REPO)
sentinel_str="[![Build Status]"
readmelines=open("README.md").readlines()
with open("README.md", "w") as fh:
    for aline in readmelines:
        if sentinel_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."