Separate tag matching and checksum lookup
Rename test
Raise ArgumentError when a symbol can't be mapped to a version

The fact that this is implemented as a hash lookup is an implementation
detail, so don't let the KeyError bubble up.
Don't raise when converting the tag to a version fails
go-appengine (386) 1.9.7
go-appengine (amd64) 1.9.7
google-appengine 1.9.7
go-app-engine-32: add 1.9.7 bottle.
go-app-engine-64: add 1.9.7 bottle.
google-app-engine: add 1.9.7 bottle.