Reset search-this-file attribute

helm-ag uses old search-this-file value, so helm-ag searches only current
file after using helm-ag-this-file. This is bug.
Remove eshell dependency

The Silver Searcher has problem on no tty environment,
like Emacs. So I implemented that ag command is executed on eshell and
I use its output because it has worked only terminal emulator
(I was not able to use 'call-process' etc).

This problem is fixed(Please see following URL for detail)
 - https://github.com/ggreer/the_silver_searcher/pull/166
Update version and update Changes