Fix Caffe2 gcc 4.8 regex issue
Fix Caffe2 gcc 4.8 regex issue

Summary:
It seems that a simple string("") conversion instead of "" is enough.
Closes https://github.com/caffe2/caffe2/pull/105

Differential Revision: D4458626

Pulled By: Yangqing

fbshipit-source-id: 5072499516332ad1067779526523a3f10aade6ef

bwasti/caffe2,pietern/caffe2,pietern/caffe2,Yangqing/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,bwasti/caffe2,sf-wind/caffe2,sf-wind/caffe2,bwasti/caffe2,sf-wind/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,davinwang/caffe2,davinwang/caffe2,bwasti/caffe2,caffe2/caffe2,xzturn/caffe2,bwasti/caffe2,sf-wind/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2