Bump Boost version on AppVeyor
Bump Boost version on AppVeyor

For VC12, use 1.58. For VC14, use 1.60 (Boost 1.58 not currently
available for VC14 on AppVeyor and Boost 1.59 has bugs in asio library).

tstein/bond,Microsoft/bond,chwarr/bond,Microsoft/bond,chwarr/bond,sapek/bond,sapek/bond,sapek/bond,sapek/bond,Microsoft/bond,jdubrule/bond,Microsoft/bond,sapek/bond,gencer/bond,gencer/bond,tstein/bond,chwarr/bond,chwarr/bond,jdubrule/bond,chwarr/bond,sapek/bond,jdubrule/bond,tstein/bond,jdubrule/bond,jdubrule/bond,tstein/bond,tstein/bond,Microsoft/bond,jdubrule/bond,gencer/bond,gencer/bond,tstein/bond,gencer/bond,Microsoft/bond,chwarr/bond