explicitly call ::bind to distinguish from std::bind

We have |using namespace std;| in this file and that seems to cause conflicts in the GCC used on the Travis CI build machines when resolving |bind|.  For instance, the Travis CI build for my push for #1211:

https://travis-ci.org/mozilla/rr/builds/29445278

Attempt to fix this by explicitly picking which |bind| we're calling.  We'll see what the build machines think of it.