Fix the inserting of /usr/local/bin into $PATH.

Summary:
The problem is that \| isn't treated as an 'OR' at all in standard
sed, you need to use 'extended regexp syntax' instead, which you do
with -E.

While in the area, I made the sed work no matter where /usr/bin
appears on the path.

Test Plan:
Manually ran the following and observed correct behavior each time:

meta.1026 [sed] khan-dotfiles% echo 'foo:/usr/bin:bar' | sed -E 's,(^|:)(/usr/bin/?(:|$)),\1/usr/local/bin:\2,'
foo:/usr/local/bin:/usr/bin:bar

meta.1027 [sed] khan-dotfiles% echo 'foo:/usr/bin/:bar' | sed -E 's,(^|:)(/usr/bin/?(:|$)),\1/usr/local/bin:\2,'
foo:/usr/local/bin:/usr/bin/:bar

meta.1028 [sed] khan-dotfiles% echo '/usr/bin:bar' | sed -E 's,(^|:)(/usr/bin/?(:|$)),\1/usr/local/bin:\2,'
/usr/local/bin:/usr/bin:bar

meta.1029 [sed] khan-dotfiles% echo 'foo:/usr/bin' | sed -E 's,(^|:)(/usr/bin/?(:|$)),\1/usr/local/bin:\2,'
foo:/usr/local/bin:/usr/bin

meta.1030 [sed] khan-dotfiles% echo '/usr/bin' | sed -E 's,(^|:)(/usr/bin/?(:|$)),\1/usr/local/bin:\2,'
/usr/local/bin:/usr/bin

Reviewers: jasonrr

Reviewed By: jasonrr

Differential Revision: http://phabricator.khanacademy.org/D3713