Insert /usr/local/bin in the PATH right before /usr/bin.

Summary:
Julia saw problems at install-time, with /usr/local/bin put at the
front of the path.  The problem is it was being put even before the
virtualenv, which means that /usr/local/bin/python was being preferred
over virtualenv python, causing any manner of problems.

The 'right' fix, which I should have done originally, was to insert
/usr/local/bin to the right place in the path, but only when it's
needed.  The 'right' place is before /usr/local/bin

Test Plan:
I manually cut-and-paste the definition of update_path() into my
shell, and ran
   $ update_path
   $ tail -n3 .profile      # I have a .profile rather than .bash_profile
   PYTHONPATH=/usr/local/lib/python2.7/dist-packages
   export PYTHONPATH
   export PATH=/usr/local/sbin:$PATH

showing it put in the /sbin addition but not the /usr/local/bin
addition, since I already have /usr/local/bin before /usr/bin in my
path:
   $ echo $PATH
   /usr/local/sbin:/home/csilvers/bin:/home/csilvers/info:/home/csilvers/khan/devtools/rbtools/bin:/usr/local/bin:/usr/local/bin/X11:/bin:/usr/local/bin:/usr/bin:/sbin:/usr/sbin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/home/csilvers/google_appengine:/home/csilvers:.

Then I did
   $ export PATH=/usr/bin:/bin
   $ update_path
   $ tail -n3 ~/.profile
   export PATH=/usr/local/sbin:$PATH
   # Put /usr/local/bin right before /usr/bin
   PATH=`echo $PATH | sed 's,\(:/usr/bin/\?\(:\|$\)\),:/usr/local/bin\1,'`

And when I did
   $ source ~/.profile
   $ echo $PATH
   /usr/local/sbin:/home/csilvers/bin:/home/csilvers/info:/home/csilvers/khan/devtools/rbtools/bin:/usr/local/bin:/usr/local/bin/X11:/bin:/usr/local/bin:/usr/bin:/sbin:/usr/sbin:/usr/bin/X11:/usr/X11R6/bin:/usr/games:/home/csilvers/google_appengine:/home/csilvers:.

which shows that /usr/local/bin is now right before /usr/bin on the
path (in addition to being where it always was, before
/usr/local/bin/X11 -- sourcing .profile undid the fake-PATH setting I
did earlier).

Reviewers: zach

Reviewed By: zach

CC: juleea

Differential Revision: http://phabricator.khanacademy.org/D2702