Issue877 vim 73 support

Issue #877 

Manual testing performed (in both 7.3 and 7.4):
- Add F# errors, check highlighted on write
- Call FsiEval with trivial request
- Call RunProject
Hi - sorry about the delay in replying.

It is a bi strange - the OnBufWritePre event handle should send a parse request that is later waited on in the CurrentErrors function (blocking) until it completes. This should just work but obviously it doesn't for some reason. It might be better to change CurrentErrors to use fsharpbinding.errors() instead of fsharpbinding.errors_current() for 7.3 as this will cause it to send a parse request synchonously.